In addition to the machinery rental fees included in the revenue account balance, the company has earned another $12,381 in unrecorded fees that will be collected on January 31, 2010.

My adjusted trial balance shows unearned rental fees as 19,924 and rental fees earned as 75,480. Do I just add earned but unrecorded rental fees to the rental fees earned? And if I do this what is the offset? Thank you!

When you accrue for earned income, the debit is a receivable and the credit is an income account. So, yes, you can increase the Rental Fees Earned account.