A Drop In pass is really a 1 visit punch card. After you set it up, it'll be ready for you to sell when you're taking attendance.
Setting Up The Pass
Create a pass (in Manage Account) called Drop In, make it a punch card with 1 visit. Optional - give it an expiration date of 1 day. (If you have a habit of giving someone a drop in class to be used later, don't set the expiration date!)
Selling the Drop-In Pass While Taking Attendance
As you're taking attendance, you'll mark the person as attending. When the "Needs Pass" flag appears, just click that red flag to sell them the drop-in pass. You'll be routed back to taking attendance as soon as you've sold the pass so you can keep taking attendance.