This isn't supported yet. People have been begging for it for a while. If you want it to be added, star the issue here:
Issue 1664 - android - Gmail should allow choosing the From: address on an account that has multiple addresses - Project Hosting on Google Code.
Also, tell everyone you know to star it. Tweet it. Facebook it. I want this feature so bad. It's just plain silly that it isn't already there.
Note: Don't comment on the issue, just star it. We want to get the devs' attention, not annoy them.