Try running MoI with elevated privileges by right clicking on the icon for it and choosing "Run as administrator" - does it work then?

By default the CustomUI wants to write files into sub-folders inside of MoI's installation folder, but also by default in Windows 7 programs are prevented from changing things under the \Program Files folder as a security measure.

It's possible to edit the CustomUI setup to write stuff somewhere else or also you can set MoI to run as administrator on startup by right-clicking the shortcut used to start it and going to Properties > Compatibility tab > check "Run this program as an administrator" at the bottom.

- Michael