Hi Alex, thanks for playing. Sorry I don't understand, Perhaps you arexpecting a bug or maybe I've given unclear directions?
The mecha defence should be purchasable on the left hand menu button (if you have enough money). They can be placed on the map with the left mouse button.
Gameplay wise they should alternate between dash attacks at the enemies and returning to recharge at the recharge station.
Please let me know, either bugs or unclear directions could be improved in a patch.