There is a bug. You can't have the Right ARM slot empty and the left one used.
Make sure you load your first ARM in the RIGHT slot.
If you load it on the left, you will have to log off and reload the game, an it will appear in the right slot.
After you make sure the first arm you load it's in the RIGHT slot, our that you have two ARMs loaded, then be aware that the Ctr+V and Ctr+C key shortcuts doesn't always work.
When they don't work, you have two other ways to change ARMS.
* Click on the View Units buttons, then the Unit button, and click the right or left units tabs. Over the unit will appear a "Manin Unit" button. You can click that to make the unit the main unit.
* Click in the level of the right or left units under the Pilot avatar. A little panel will appear. If the unit is OK, it says "Main Unit", "Close", "Peace" and "Close".
- Main Unit is to change the unit to main.
- Close is a mistake, it should say "Attack". It sets the unit in aggressive mode.
- "Peace" makes the unit stop attacking when you are attacked or attack something.
- Close closes that panel.
If the unit is broken, it will show a little message explaining the time it will be repairing and how to repair it faster, and a button to reduce the repair time to 2 minutes, if you have a repair kit.
If those methods doesn't work, then log out and relog.
If that doesn't work, close the game completely and open it again.
If that doesn't work either, then there must be something wrong with your account, and you'll have to send your account name to sage0228.