Can someone explain to me how to post an adjustment to a prior year?

If we are talking about posting an adjustment batch to the prior year in the general ledger, then

1. Enter the batch of transactions
2. Select Post | To prior year from the menu

You might want to check out the F1-Help on the topic.

What about the date? All I can enter is this year's date.

Under Edit Company profile | Batch Options, what is the setting for "Validate closed periods", and is your prior year marked as "Closed"?

Having said that, the date of the transaction is irrelevant in the general ledger. The transaction is posted to the year and period you specify, regardless of the date of the transaction.