Can't find MonoDevelop. Unity 4.5.2

It’s not opening when I double click on a script and the monodevelop folder is not in the Unity installation folder.

Yes, I selected to install MonoDevelop during the installation.
It’s selected in the External tools (MonoDevelop(Built-In)).

Please help me, this is urgent. I need to finish my project.

OS: Windows 8.1 Update 1

Don’t choose build in, instead try browse and find ProgramFiles (x86)/mono develop/bin/monodevelop.exe