Step 1 is to buy the 7.4 upgrade.
You then need to update to 7.4.2 (free) and it will work properly with Leopard.
Download it here:
Digidesign | Support | Pro Tools 7.4.2 Download for Mac OS X 10.5.3 & 10.5.4 Leopard
(You don't even need to install 7.4 first)
You'll need your 7.4 serial number to activate it.