how do agents get short listed and then interwied( the foudation cannot just put an ad in the newspaper)
I've always assumed that the Foundation recruits from existing organizations that are top-tier but not overly in the public eye (so for example, researchers would be pulled from cutting-edge science labs, big corporations working with multimillion-dollar tech, or certain academic institutions like universities). Basically, you want a good balance between documented skill (because the Foundation wants the best of the best who won't need too much new training) and relative anonymity (there's waaaay too much cover-up needed if someone super-famous suddenly stops communicating with their usual coworkers/stops collaborating with their research associates).
what happens after the foundation contacts a d-class with an offer and they refuse?(i am just assuming class-x amnesiacs)
Amnestics might be used, but I'm sure the Foundation would have some sort of covert way of questioning prospective D-Class that is vague enough not to require extensive cleanup should there be a refusal. (I personally don't think they'd lead with "Hey, so these things that aren't supposed to exist, wanna go test with them?" I think it'd be more like "we're offering you a chance to live.")