To manually add one GL account to a Fund follow these steps.  This assumes the GL account has already been added to the Master Chart.  You cannot add a GL account that is not listed in the Master Chart..
  1. Go to the General Ledger > Accounts > Account Tab.
  2. Click New.
  3. Type the Fund Id in the 5th box (the first large one). 
  4. Enter the natural account in the 6th box.
  5. Click into the next box (or hit Tab) and this will cause the Account Type and Description to populate.
  6. Confirm that these are correct or change if necessary.
  7. Confirm or change the fiscal year.
  8. Save the record to finish.
To easily add additional accounts for the same fund for the same year, click the copy button and change the natural account. Similarly you can add additional accounts for different funds.

To automatically add all accounts to a fund based on the it's class, refer to How to add multiple accounts to a fund