The simplest way to unlock more units is to clear blocks. Once you're finished Block 8, more units can be unlocked via different criteria like clearing Secrets (by destroying the secret unit that appear after certain conditions are fulfilled), Ace ranking missions and clearing sub-missions. Do note that each secret, ace ranking and sub-mission will only be counted ONCE, so you won't be able to replay a mission again and again to ace rank it a few times to get your ace rank count up, for example, despite the statistics page showing how many times you clear a secret, ace rank a mission and/or sub-missions cleared.