Running on: Snow Leopard
Scanner: CanoScan LiDE90
You install the drivers to your scanner and it's not showing up on the File>Import list.
Go here and download this file:
Once you've downloaded the file, click on the file for your language, then go to:
Goodies>Optional Plug Ins>Import-Export>TWAIN.plugin
Take this "TWAIN.plugin" file and drag it into your existing Photoshop CS4 folder in your applications folder in that same format: (so Goodies>Optional Plug Ins>Import-Export> drop the TWAIN.plugin here)
THEN you download your scanner drivers (even if you've downloaded them already, re-download them), restart your computer, restart Photoshop, and it should be able to see your scanner now!
If you wanna read up again on how to do it, go here: