Okay what I used was...
-My FireRed game on my GBA SP
-My Pokémon game on my GC
I have linked the link cable between the game player and the GBA SP. Only the link cable for GBA. Nothing else. I was at Phenac Center & Cerulean Center, at the right place too. Then, on my FireRed, it said: ''Waiting.... press B to cancel''. Sounded pretty normal to me. But then on my TV screen where I was playing XD, it showed a picture of a GBA with a controler and another controler linked via the GBA SP. How do I get that think?