Is it that you cant use them or that you cant find them when you select your units? If its the later, then on the page were you select your units there are tabs above the units to click for more unit options
1st tab is your barracks produced troops which is the default tab when you enter the fight screen
2nd tab is your training ground produced troops -- you will find your Cerberus there
3rd tab is your Mercenary Troops
the 3 tabs are right above the current default pictures of your units and look like shields with crossed weapons
Hope this helps and good hunting