How do you capture tiles in Doomsday?

You have to first build your processing plant and virus research facility next to your allience center. Then tiles can only be occupied next to the turquoise border plots your allience members have already occupied or green border tiles you've previously occupied. You have to start with a level 1 tile, then work your way up to higher level tiles, much like you have to work your way up when killing zombies. You won't be able to occupy tiles with polluted rss until you've upgraded the virus research facility enough to handle the infection. To occupy just click on the file and select "occupy" then an APC will martch to the tile and take it, if it can.