So I was using BellyZard on PBR and it has a 30 IV stat in HP which is what the Smogon analysis said to put. But when I did the Sub and then used Belly Drum, it didn't activate. Is this b/c it only works with Charizard at lvl 100 or what?(PBR sets it to lvl 50)
Yes. Your Hp needs to be divisible by 4.
At level 50, Charizard with no Hp EV's and a 30 IV hits 153 Hp, if my maths is right.
The analyses are tailored to level 100 pokes. How much Hp does your level 50 Charizard have? You might need like 12 EV's or something to make the berry activate.