Install iTunes on your main computer before trying to use your iPhone/iPad/iTouch.
If you upgrade to RTM Pro and configure your mobile device correctly, the RTM iPhone/iPad app syncs automatically with Remember the Milk. (If you don’t go Pro, you can sync manually any time by tapping the Sync symbol at lower left.)
No syncing is necessary if you access your Remember the Milk tasks at m.rmilk.com. You will need an active cellular data connection.