Answer 1

It learns seismic toss in FireRed/LeafGreen, by Move Tutor. So you would do that then transfer it to Diamond/Pearl/Platinum then to your Black or White.

Comments

Edited your answer, Blissey learns ST as well.

Answer 2

It doesn't... In Gen 4 & 5

However, in Gen 3, it learns it by Move Tutor