1. Edit the Settings.ini file (located in the CPS installation folder) by adding these lines:
customview=1
cpsverview=1
They will enable the developer menu switch by itself. But selecting it will produce an error — that's what we need the next steps with developer.usr
2. Try opening your codeplug. The CPS will tell where you have to put the developer.usr file to.
3. The path in question partially resides within zip archives, but Windows can handle them natively.
4. Put the developer.usr file to that path.
5. Now you should be able to enable the developer mode.
NB: Due to the forum restrictions the attached file has .txt extension. Rename it to developer.usr after saving.