Force newline in concepts

Assuming you want every new concept to start at a new line, you can do like this.

\documentclass{sig-alternate-05-2015}
\usepackage[english]{babel}

\usepackage{etoolbox}
\makeatletter
\patchcmd{\ccsdesc@parse}{\textbullet}{\par\noindent\textbullet}{}{}
\makeatother

\begin{document}

\begin{CCSXML}
<ccs2012>
<concept>
<concept_id>10002951.10003227.10003351.10003445</concept_id>
<concept_desc>Information systems~Nearest-neighbor search</concept_desc>
<concept_significance>500</concept_significance>
</concept>
<concept>
<concept_id>10003752.10003809.10010055.10010060</concept_id>
<concept_desc>Theory of computation~Nearest neighbor algorithms</concept_desc>
<concept_significance>500</concept_significance>
</concept>

</ccs2012>
\end{CCSXML}

\ccsdesc[500]{Information systems~Nearest-neighbor search}
\ccsdesc[500]{Theory of computation~Nearest neighbor algorithms}

\printccsdesc
\end{document}

A horrible hack for just placing a new line command before the second item:

\documentclass{sig-alternate-05-2015}
\usepackage[english]{babel}

\newcommand\HH{% horrible hack
  \global\let\savedtextbullet\textbullet
  \gdef\textbullet{%
    \par\noindent\savedtextbullet\global\let\textbullet\savedtextbullet
  }%
}

\begin{document}

\begin{CCSXML}
<ccs2012>
<concept>
<concept_id>10002951.10003227.10003351.10003445</concept_id>
<concept_desc>Information systems~Nearest-neighbor search</concept_desc>
<concept_significance>500</concept_significance>
</concept>
<concept>
<concept_id>10003752.10003809.10010055.10010060</concept_id>
<concept_desc>Theory of computation~Nearest neighbor algorithms</concept_desc>
<concept_significance>500</concept_significance>
</concept>

</ccs2012>
\end{CCSXML}

\ccsdesc[500]{Information systems~Nearest-neighbor search\HH}
\ccsdesc[500]{Theory of computation~Nearest neighbor algorithms}
\ccsdesc[500]{Nothing particular~Just for testing}
\printccsdesc
\end{document}