Bulleted lists in moderncv cventry

You can try the \mybitem command defined like in the answer you linked to, but with the lengths now properly calculated:

\documentclass[11pt,a4paper]{moderncv}
\moderncvtheme[blue]{casual} 
\usepackage[utf8]{inputenc}

\usepackage[scale=0.8]{geometry}
\recomputelengths

\firstname{first}
\familyname{last}
\address{123 First St}{Somewhere, CA 90210}
\mobile{555.555.5555}
\email{[email protected]}
\extrainfo{info}

\newlength\mylen
\addtolength\mylen{\linewidth}
\addtolength\mylen{-\hintscolumnwidth}
\addtolength\mylen{-\separatorcolumnwidth}
\addtolength\mylen{-3mm}

\newcommand\mybitem[1]{%
   \parbox[t]{3mm}{\textbullet}\parbox[t]{\mylen}{#1}}

\begin{document}
\maketitle

\section{Experience}

\cventry{years}{degree/job title}{institution/employer}{localization}{grade}
{\mybitem{some bulleted entry}
\mybitem{some bulleted entry which is so long that it will span more than a single line, or at least, that's what I hope}
}

\closesection{} 

\end{document}

However, why don't you use a modified itemize environment?

\documentclass[11pt,a4paper]{moderncv}
\moderncvtheme[blue]{casual} 
\usepackage[utf8]{inputenc}
\usepackage{enumitem}
\usepackage[scale=0.8]{geometry}
\recomputelengths

\firstname{first}
\familyname{last}
\address{123 First St}{Somewhere, CA 90210}
\mobile{555.555.5555}
\email{[email protected]}
\extrainfo{info}

\begin{document}
\maketitle

\section{Experience}

\cventry{years}{degree/job title}{institution/employer}{localization}{grade}
{\begin{itemize}[label=\textbullet]
  \item some bulleted item
  \item another bulleted item, this time so long that it will span more than one line, let's add some more text .
\end{itemize}
}

\closesection{} 

\end{document}