If you need to enter an adjustment where one side of the entry goes to an account that it not included in the current trial balance you will need to set up a new code in the TB so that the journal can be coded to the correct place:
To manually add a line into the trial balance, use the 'insert' function on the item above where you want the new account (from the green arrow that appears when you hover over the trial balance item)
Then enter the name of the account and code (optional). You will then be able to post journals to that page.
Or you can chose 'manual entry' under work items on the trial balance input page then drag and drop the new account to the correct place.
Now you may create a journal entry to that new account.
Also see Creating Journal Entries/Adjustments