From the main screen, goto 'Snapshot | View Record'. If the record in question is not already displayed, you need to find it. Ctrl+F12 will go to the last record in the database. From there, press F11 as necessary until you locate it. (this assumes that you haven't sorted the snapshot database). Press Ctrl+D to mark the record as deleted.
If you are writing a sequence and just entered an incorrect record, from the menu bar, select 'View | Last snapshot record used'. Press Ctrl+D to delete it.