If you're using the NLSC roster update, Allen Iverson isn't in the active All-Star rosters because he sat out the game this year. I left him on the team on the inactive list though, so you can activate him and use him in All-Star Weekend simply by reordering the rosters.
If you're using another roster update with updated All-Star rosters, the creator of the patch may have removed him from the All-Star rosters completely. It's easy enough to
add him again.
If you're referring to the All-Star Weekend in Dynasty Mode, he's probably not been selected because the CPU is prone to making really strange decisions when it selects the All-Stars.