Consider the following example:

\documentclass{article}
\usepackage{undertilde}
\begin{document}
$\utilde{x}$
\end{document}


Whenever I am trying to compile it with Kile, shows the error:

undertilde.sty not found.


I have trying to install it from ubuntu14.04(32bit) software centre by searching the name 'undertitle' but there are no such file. How can I solve this problem?

The package undertilde is marked as having an unclear license. As such, it can't be included in TeX Live (which only includes free software). You will have to download the package from CTAN and install it manually: see How do I install an individual package on a Linux system? for more on this (look in particular at installing a package manually).