You'll need a custom device handler to add the Double Switch correctly and have full control of the dimmer and relay part of the device. 

If you haven't added handlers before, just follow the step-by-step instructions below and you'll be done in 5 minutes:

1. Exclude the Double Switch (if you already included it to ST).
2. Log in here with the same information you use for your SmartThings app.
3. Click on My Device Handlers in the top menu.
4. In the top right corner, click on "Create new device handler".
5. Choose "from code" from the top tab menu. You'll be presented with a blank area where you can paste the code from the following link.
6. Please click on this link to access the code for the handler, then copy ALL of the code from the page.
7. Next, scroll to the bottom and click Create.
8. Now click Save, then click Publish > For Me.

You should see this device type in your list of device handlers.

Now add the Double Switch to your system:

• Tap the plus icon in the home screen
• Tap Device
• Tap "Scan nearby" (in the corner of the screen)
• While the app says "Scanning...", tap the upper paddle of the dimmer 3 times as quickly as possible
• The app will indicate when the device connects
• Tap Edit to rename the device, and tap Done to save the name
• Tap Done to complete the setup

If you're having issues with adding the switch correctly to your hub, try excluding the device using the steps shown here. Exclusion acts as a reset and it can help with a variety of configuration issues.

If exclusion doesn't help, let us know! We'll be happy to assist.