you can go after you beat the game. same place. it will keep sending you back to root town after each one but just go right back in and you can get the next one.
If you guys have any problems playing the games (ie. difficulty finding the memories) then you should visit the faq sheets on the main site. Here is the walkthrough for //Mutation. http://www.dothackers.net/mutationwalkthrough.txt The tips for finding the memories are near the bottom.