Select your Asset Tracker product and then the Devices icon. Click the Add Devices then Add One Device. Copy and paste the Device ID (24-character hex). You could also type the serial number in this box.
Adding the device to your product will automatically add the SIM card to the product as well.
If you want to be able to flash the device directly from Workbench, CLI, or the Web IDE, you will want click the ... button on the right edge of the device list for your device and select Mark development device.