Hi bleearg,
just a question: Is the problem permanent? I assume your NPM version is NPM 10.3.1 or older (in NPM 10.4 and newer this shouldn't happen). It happens because when you hit the + first time, the add resource page is being compiled. This means if you try to repeat the action after several minutes, the page should load properly. Since NPM 10.4 the website precompilation is performed during Configuration wizard (after installation phase).
If you are still experiencing the same problem and have no luck with adding resource, please open the support ticket for deeper investigation of the problem.
Thanks,
Honza