Here are two possible solutions. The first is kind of minimal, in that it uses an align* environment and provides for two line breaks. (Well, I also replace most instances of \times with \cdot, and I eliminated a spurious ) in the middle row.)
The second solution goes further towards making it easy on your reader to actually read and comprehend the material. E.g., by writing \frac{1}{m} at the beginning of each of the three additive terms, the remaining material can be typeset using larger lettering. After all, the material that's being summed up is at least as important as the \frac{1}{m} term, right? The second solution also gets rid of all \cdot directives and places the 100\% block at the start, to give it more visual prominence.

\documentclass{article}
\usepackage{amsmath} % for 'align*' env.
\begin{document}
\begin{align*}
\mathit{PaD}^{i}
&=\biggl[ \frac{(1-\frac{n_{1}}{N})\cdot M(i,1)}{m}\\
&\qquad+\frac{\sum_{t=2}^{m}[(1-\frac{n_{t}}{N}\cdot \frac{n_{t-1}}{N})\cdot M(i,t)\cdot M(i,t-1) ]}{m}\\
&\qquad+\frac{\sum_{t=2}^{m}[(1-\frac{n_{t}}{N}\cdot \frac{N-n_{t-1}}{N})\cdot M(i,t)\cdot (1-M(i,t-1))]}{m}
\biggr] \times 100\,\%
\end{align*}
\begin{align*}
\mathit{PaD}^{i}
=100\,\% \times \biggl\{ \phantom{{}+{}}
&\frac{1}{m}\Bigl(1-\frac{n_{1}}{N}\Bigr) M(i,1)\\
+{}&\frac{1}{m}\sum_{t=2}^{m}\Bigl[\Bigl(1-\frac{n_{t}}{N} \frac{n_{t-1}}{N}\Bigr) M(i,t) M(i,t-1) \Bigr]\\
+{}&\frac{1}{m}\sum_{t=2}^{m}\Bigl[\Bigl(1-\frac{n_{t}}{N} \frac{N-n_{t-1}}{N}\Bigr) M(i,t) (1-M(i,t-1)) \Bigr]
\biggr\}
\end{align*}
\end{document}
\frac{1}{m}term outside the curly braces. However, I didn't go that far since I don't know if the meaning of the three additive terms depends on them being scaled by\frac{1}{m}. For sure, additive terms 2 and 3 seem to be some kind of average... – Mico Feb 16 '17 at 07:53splitas the inner environment. – Mico Feb 16 '17 at 08:35ansmathpackage. – Mico Feb 16 '17 at 08:39alignedforalign*and wrap the whole thing in\begin{equation} ... \end{equation}. (in this situation,alignedis very similar tosplit, which Mico has already suggested.) – barbara beeton Feb 16 '17 at 14:58equationas the outer environment andsplit(oraligned-- see barbara's comment) as the inner environment. – Mico Feb 16 '17 at 22:50