did you follow the steps off the customize forum?
1. install the SUID fix, if you are on firmware 1.1.3 or Later (listed under tweaks 1.1.3)
2. install summerboard
3. launch summerboard, close summerboard
4. reboot the phone
5. install customize 2.0 from installation source
6. launch customize, close customize
7. launch summerboard, set the theme to "customize", ensure both "theme wallpaper" and "wallpaper" are ON all others OFF, close summerboard
8. launch summerboard and confirm that your settings are correct as mentioned in step 7, close summerboard
9. launch customize, download the theme you want, apply it to the phone, exit/respring and you should now see your theme installed