The next lot being sold will appear next to a check box in the lot selection area on the right of the screen. If a choice group is being offered, multiple lots will be listed.
If you want to place a bid on an item listed, select the check box to activate your bid button. If you're bidding on a choice group, select the check box next to your first choice. Only select one lot, even if you want to buy more than one item in the group.