you're looking at a soldier unit you'd like to use but it is locked, hover your mouse over it. look for what upgrade you need to purchase to unlock it and, if you have enough money, purchase that upgrade. one important note, though - take the cost of the upgrade into consideration as well as the cost of the unit. if purchase the upgrade takes all of your money, you won't have enough money left to train the unit which is clearly not a good thing. instead of spending all of your money on the upgrade, send out some of the units that are already unlocked and use them to 