This is from another forum, but I'm the one who posted it so I don't mind.
This will show you how to install apps and games on your phone that you have an existing .apk file for using Astro File Manager.
1. Go to the market on your phone and search for Astro File Manager and download it.
2. Plug in your phone pull down your notification bar.
3. Press "Select to copy files to/from your computer".
4. Press "Mount"
5. Now go to my computer on your desktop and open up your sd card (removable disk G: on my computer)
6. Create a folder named apks(or whatever you want it to be) and put the .apk file from your computer in this folder.
7. Now close the window on your computer.
8. On your phone pull down your notification bar and press "Turn off USB storage".
9. Open up Astro File Manager on your phone and go to the folder that you created.
10. Press on the app you want to install.
11. Press "Open App Manager"
12. Press "Install"
13. The App Manager will run and then press "Install" again.
14. *Your app will now install, when it is done you can either open it up from there or press done and exit out of Astro.
Your app that you just installed will be in the list of "All Programs".