That's fine to ask here on Super User, but before you ask:
- Collect the version information for your OS (Linux Mint) and Monodevelop, and include it in your question. Very important!
- Tell us how you installed MonoDevelop, including the exact steps, and if you downloaded packages from a repo, whether it was the standard Linux Mint repo or a custom one.
- Tell us exactly what doesn't work, and provide screenshots if possible.
- If possible, provide a reference to somewhere that suggests that this feature should be available for C++ specifically.
- Tell us what you've tried to fix it.
- Tell us whether you've upgraded Linux Mint or Monodevelop from one version to another without reinstalling
- Tell us whether you've tried deleting your monodevelop user directory (not sure exactly what it would be, but the logical choice would be
~/.monodevelop or something similar).
We need: details, details, details, and what you've tried in order for this to be a good question.