VST2,X Path - greyed out!

Hi, apologies if this has been raised before - I’ve done a quick search but can’t find the answer.

I need to add some different vst paths to Cubase 5 for an old project that I need to re-establish. It’s something I’ve done many times before, but for some reason the button to do this in VST Plugin Manager is greyed out?
Can anyone help me figure out how to re-enable this option because I really need to open this old session urgently!!

I’m running Cubase 5.1 in Windows 7-64bit.