Is it true that f(x) fanclub name is PLUS??

No, PLUS is not f(x)s fanclub name. f(x) Sulli sent out a me2day to Jiyoung saying "plus" i'll see 你 later and people 说 that koreans don't use plus so she had to be talking to the 粉丝 indicating a fanclub name. She later stated that PLUS was NOT the fanclub name and, even if she wanted it to be SM has not made any official statement.

There is no official fandom name for f(x). It was supposed to be out at Pinocchio era but f(x) and SME weren't happy with it. f(x) fans, i think... is still in the miffle of brainstorming until now. 或者 probably, f(x) will come out with their fandom name on their comeback this year.

--That's from what I know.

No it is "Aff(x)tion" ^^

Look here: link

I think it is VALSK, isn't it?

PLUS is actually the official 粉丝 base name, I've done research and found out through SM myself that it is. (And a bunch of different sites as well)