that's the thing - there is no 'apply release' button. Just the selector box (or upload) then a 'submit' button.
1. "Apply release" link (not button, sorry).
2. List of already attached releases.

Note: you cannot do this in bulk mode, only image by image.
hmm.. perhaps MikLav's advice is to be taken. It is not worth spending much time figuring it out.

FP made me more than: LuckyOliver, Zymmetrical, Crestock, Cutcaster, MostPhotos, YAYmicro. It's the fastest site to upload to after MostPhotos and YAYmicro.
In its good days, it gave about one download per 2-3 weeks for me (7$ each). Begin this year, it virtually went dead. There seems to be a very slow recovery now but I wouldn't upload a full port any more from scratch.