Computer asset should be a fixed asset type account
open the register for the loan account, make a new first entry, or edit what is there, the full amount of the purchase agreement should be entered as an increase, and select the computer fixed asset account in the account block and save
when you make payments, you use the loan account as the expense for the payment, check or credit card payment.
Journal entries should be the exception when using QB, they often do not work as you think they should, and they never work for inventory, when you use inventory type items. It is much better to use the forms on the home page the way QB is designed to be used.