It's very easy to transfer money from one account to another. Simply add a transaction as usual, and in the Category box click on the popup arrow button and a list will appear, and select the account you want to transfer the amount to. You will find all the transfer accounts at the top of the list prefixed with “Transfer”. For example, if you want to transfer to your Credit Card account, you would select “Transfer:Credit Card” (or whatever the actual name of your credit card is).
The same also applies to adding transfers for reminders and split transactions. Simply select the "Transfer:Account Name" option from the category list when adding a reminder or transaction split.