The select key will select anything the cursor is hovering over. The enter key will select anything with a highlighted yellow box around it (for example: when you use the navigation arrows). It will also act as a normal enter key on a keyboard.
For a more technical explanation: The air mouse and keyboard are seen by Android as two separate devices, a USB mouse and a USB keyboard. Since they are technically two different devices they have their own select keys. The SEL key is a left mouse click, and the enter key is the same as a keyboard's enter key.
Important: Mohu Channels has been discontinued and is no longer under warranty. Some third party retailers may still sell Channels units, but they are not covered by any Mohu warranty.
Please contact the reseller if you have any questions.