Use the Produce option to create more troops. You need more or equal troops to the place you are attacking to be effective.

The guy above me is incorrect. The icon with the dumbbell is the game's way of saying you should increase your Kaiju's power before attacking this city. Usually by training.

You should pretty much always be at or near max strength and defence before attacking the next city. (the maximums increase after each promotion)

There's no reason to ever hit the Training button. If your kaiju power is low, it means you aren't having sex with Daigneux like you're supposed to do, every single day. It's the fastest way to max out your stats, especially if you make her octuple orgasm.