To check attendees into a session you need to enter the Session Experience first. To do so, pull up the guest list for the event and tap the Kiosk icon at the very top.
Tap on the "Session Experience" option.
A list of your sessions should appear. Choose which session you'd like to start scanning guests into.
The session experience screen should provide you with a small window to scan QR codes and barcodes. To check guests into the session you have to make sure that the "Check-In" option is highlighted as shown below:
When scanning a QR code or barcode, you should receive an image similar to the one below.
When checking someone out of a session, tap the lower bottom right corner so that "Checkout" is highlighted.
Once you've checked a guest out of a session a screen similar to the one provided above should appear, however this message will be outlined in orange as seen below.