Both mono and xbuild can be found on AlexT's Repo. If you don't have it set up, follow the steps in the link above. If you have already set it up follow these steps:
opkg update opkg install mono opkg install mono-xbuild
It was helpful.
But compilation has to be done with following command:
>>mono path to xbuild.exe path to .sln
xbuild.exe can be found in the following directories:
/usr/lib/mono/4.5/xbuild.exe /usr/lib/mono/xbuild/12.0/bin/xbuild.exe /usr/lib/mono/xbuild/14.0/bin/xbuild.exe
I found them using the find command: find / -name xbuild.exe.