Hey Gwen,
Entries to A/R must be done as receivable charges through the regular transaction entry screen. Credits are a negative receivable charge and debits are a positive receivable charge. Offset that entry with an A- line to Prepaid Items. Once that has been done, go and do the General Journal entry as instructed, simply replacing the account for A/R with Prepaid Items account number.
The reason that A/R cannot be entered through the General Journal screen is that when posting to A/R we are saying someone either owes us money or possibly owes us less money. The Transaction Entry screen allows you to say who that person is, whereas the General Journal entry screen does not have that facility.
