The simplest method with tikz :
First I defined a point (LegendBox_anchor), it's an anchor for the legend's box.
Then I used a line to show the color used for a tkzKiviatDiagram Line. The different lines are placed with (LegendBox_anchor). The description is a simple node add at the end of the line. anchor=west is used to align correctly the descriptions
\coordinate[xshift=-2cm] (LegendBox_anchor) at (current bounding box.south east) ;
\draw[line width=3mm,color=red] (LegendBox_anchor) -- ++(.5,0) node[anchor=west] {red description};
\draw[line width=3mm,color=blue] ([yshift=8mm]LegendBox_anchor) -- ++(.5,0) node[anchor=west] {blue description};
\draw[line width=3mm,color=green] ([yshift=16mm]LegendBox_anchor) -- ++(.5,0) node[anchor=west] {green description};
It's possible instead of lines to use rectangles or nodes. The nodes have several advantages to get some refinements but I tried to create something easy to use.
Another possibility is define a box around the different description. I can if you add this possibility later.
Now it's perhaps interesting to add a macro to the package to get legends. Here I added a simple macro ( first try to extend the package )
\newcommand{\LegendBox}[3][]{%
\coordinate[#1] (LegendBox_anchor) at (#2) ;
\foreach \col/\item [count=\hi from 0] in {#3} {
\draw[line width=3mm,color=\col] ([yshift=\hi*8mm]LegendBox_anchor) -- ++(.5,0)
node[anchor=west] {\item}
;}
}
It's based on the first lines. Argument #1 is used to get a better position of the legend box if it's necessary.
\documentclass[]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[upright]{fourier}
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{tkz-kiviat,numprint,fullpage}
\usepackage{pgfplots}
\usetikzlibrary{arrows}
\pgfdeclarelayer{background}
\pgfdeclarelayer{foreground}
\pgfsetlayers{background,main,foreground}
\newcommand{\LegendBox}[3][]{%
\coordinate[#1] (LegendBox_anchor) at (#2) ;
\foreach \col/\item [count=\hi from 0] in {#3} {
\draw[line width=3mm,color=\col] ([yshift=\hi*8mm]LegendBox_anchor) -- ++(.5,0)
node[anchor=west] {\item}
;}
}
\begin{document}
\begin{tikzpicture}[label distance=.15cm,scale=0.75]
\begin{scope}[rotate=30]
\tkzKiviatDiagram[radial=3,lattice=7,gap=1,step=1,label space=2]%
{Cover,
Droppings,
Other,}
\tkzKiviatLine[thick,color=red,fill=red,label=SiteA](0.78,5.59,2.02)
\tkzKiviatLine[thick,color=blue,fill=blue](5.92,1.57,3.06)
\tkzKiviatLine[thick,color=green,fill=green](2.9,4.6,3.6)
\tkzKiviatGrad[suffix=\%,unity=10](0)
\end{scope}
\LegendBox[xshift=-2cm]{current bounding box.south east}%
{red/red decription,
blue/blue description,
green/green }
\end{tikzpicture}
\end{document}

remark : you don't need to add ; at the end of \tkzKiviatLine
Update :
I added a box around the legends, removed some ; I used nodes instead of lines
\documentclass[]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[upright]{fourier}
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{tkz-kiviat,numprint,fullpage}
\usepackage{pgfplots}
\usetikzlibrary{arrows,fit}
\pgfdeclarelayer{background}
\pgfdeclarelayer{foreground}
\pgfsetlayers{background,main,foreground}
\newcommand{\LegendBox}[3][]{%
\xdef\fitbox{}%
\coordinate[#1] (LegendBox_anchor) at (#2) ;
\foreach \col/\item [count=\hi from 0] in {#3} {
\node[color = \col,draw,
fill = \col!50,
minimum width = 4 ex,
minimum height = 2 ex,
label={[anchor = left,name=b\hi]right:\item}] at ([yshift=\hi*4 ex]LegendBox_anchor) {};
\xdef\fitbox{\fitbox(b\hi)}
}%
\node [draw,fit=\fitbox(LegendBox_anchor)] {};
}
\begin{document}
\begin{tikzpicture}[label distance=.15cm,scale=0.75]
\begin{scope}[rotate=30]
\tkzKiviatDiagram[radial=3,lattice=7,gap=1,step=1,label space=2]%
{Cover,
Droppings,
Other}
\tkzKiviatLine[thick,color=red,fill=red,label=SiteA](0.78,5.59,2.02)
\tkzKiviatLine[thick,color=blue,fill=blue](5.92,1.57,3.06)
\tkzKiviatLine[thick,color=green,fill=green](2.9,4.6,3.6)
\tkzKiviatGrad[suffix=\%,unity=10](0)
\end{scope}
\LegendBox[shift={(-3cm,3cm)}]{current bounding box.south east}%
{red/red decription,
blue/blue description,
green/green description }
\end{tikzpicture}
\end{document}

Other,? because the result is different withOther. – Alain Matthes Apr 07 '13 at 07:25