So I downloaded some addons using curseforge (had to download 3.3.5 v addons to my ptr folders and then copied them over into interface/addons/[addon name]) and most don't work at all. As a matter of fact the only one that does is addon control panel, funny enough.
At the character select screen, there should be an "AddOns" button in the lower left corner. Click on that. If any of the mods in the list that pops up say "out of date" or some such, choose the "Load Outdated AddOns" option.
It says one is incompatible, even though the version I downloaded is for 3.3.5. All the others don't give any kind of warning. But none work except for Addon Control Panel and Skillet