On zero editor, when I place a vehicle spawn object and I type in what vehicles I want to spawn, like imp_walk_atst. I don't get the ATK and DEF classes because I typed in imp_walk_atst in impATK and impDEF and they don't spawn in ant of the classes. I'm getting confused.

Inserting a vehicle for the ALL(alliance side) should fix the prob. Make it something weak like a speederbike if u want. Make sure u inserrt in ALLATK and DEF too

Go reread the docs, as it's explained there. If you have that as you wanted (I believe you have it right), and it's still not showing, check your memorypools in the lua. Walkers require an extra step to get them ingame.