Sadly, there's no way of a direct use of xls files in LaTeX, more precisely the OLE 2 Compound Document format, which is in binary form.
From 2007 on, Microsoft Excel uses Office Open XML for their new xlsx format, based on XML. But I don't believe it would be easier to parse it from LaTeX. Update: format is zipped, so binary again.
My suggestion is to use an easier format, say, comma-separated values. cvs is a plain text format. Excel can easily export a spreadsheet to csv.
We have the datatool package which handles csv files and might help you to populate your table. I'm not sure how it would work with TikZ.
Joseph also suggested me that pdfplotstable might help with displaying the data.
If you do want to stick with xls files, Werner's suggestion is the way to go.
Hope it helps. :-)
Update: Here's an example with a plain and simple table:

When saving it, I can choose the csv format:

Which will give me the following plain text (scores.csv):
ID,Name,Score
1,Knuth,10
2,Lamport,10
3,Mittelbach,7
4,Oetiker,7
Now, in LaTeX with datatool:
ex1.tex
\documentclass[a4paper]{article}
\usepackage{datatool}
\begin{document}
\DTLloaddb[keys={ID,Name,Score}]{scores}{scores.csv}
\begin{table}[htbp]
\caption{Our final score}
\centering
\DTLdisplaydb{scores}
\end{table}
\end{document}
ex2.tex
\documentclass[a4paper]{article}
\usepackage{datatool}
\DTLloaddb[keys={ID,Name,Score}]{scores}{scores.csv}
\begin{document}
\begin{table}[htbp]
\caption{Our final score}
\centering
\begin{tabular}{clr}
\textbf{ID} & \textbf{Name} & \textbf{Score}
\DTLforeach{scores}{%
\id=ID,\name=Name,\score=Score}{%
\\ \id & \name & \score}
\end{tabular}
\end{table}
\end{document}
The output:

(I don't know how to do that with TikZ, sorry.)
There's a follow-up to this question: Can luaLaTeX convert xlsx tables into LaTeX code?. With LuaLatex, it might be possible to unzip the file and extract data from the resulting XML. Not so easy though, IMHO.
xlsor other OLE 2 Compound Document format. If you want to automate your table generation, I strongly suggest you to export thexlsspreadsheet to acsvformat and then read it in LaTeX using thedatatoolpackage. I have no idea how to integratedatatoolwithTikZ.:-(– Paulo Cereda Oct 05 '11 at 19:00