This folder will be used by the attach binary. Make sure you have both the Source and OrangeBox binary in it.
Here are some instructions on how to prepare an upcoming release:

1) Copy the latest MM:S dlls (Source+OrangeBox) and "hl2launch.exe" into this directory
2) Attach .source and .orangebox to the particular binaries.
3) Run Attach.exe
4) Test MMS_Installer.exe once (should work but nobody wants a buggy installer)
5) If everything worked fine, release it, otherwise pm me (Basic-Master)