Hi goldendunner,
To activate the formation's tab, you first need to ensure that you have the units you wish to form up selected.

Once you have done this, click this button on your R:TW GUI.



A large number of formation options will slide onto your screen. If you hover your mouse over one of them, you will see a brief description. Simpy select the one you wish to activate and your units will begin moving into position.

Hope this helps, good luck, cheers!