My commander left and promoted me to Commander. She gave me her email and password. How do I access..

Make sure your current game is bound to an IM30 account so that you can get back to it. It must say 'binding successful'.

Then go to the Settings menu -> Account and then to on 'Switch Account'. Here you can log into the other game. Assuming the details you have are for an IM30 account take that option and then log in. This will load the other game for you to access and play as normal.

You can then go back to this option when needed and do the same process to log back into your game. Once you have logged in to each it should remember the details and make switching easier.