Just follow the steps that @JDRoberts outlined in his excellent tutorial. If you actually do the steps as you read the tutorial it is much more obvious on what to do. Since you haven’t gotten your devices yet…
DT5 says “select MY DEVICES in the IDE and choose the specific device you want to have use that new device handler.” So at this point you should be in the MY DEVICES not MY DEVICE HANDLERS.
So here is a screen capture of the MY DEVICES , and the second column is the Device Type column. After you pair up your WD100+ it will probably have something like “Enhanced Dimmer Switch” in the Type column and you need to edit that to be the custom device handler “WD100+ Dimmer” by @Darwin
like you see in my screen capture. So to do that click the name you gave your switch where I highlighted mine in yellow.
That pulls up this screen so you can edit the attributes of the Device itself and the one you want to edit is accessible via the drop down box for TYPE to switch it to “WD100+ Dimmer”. Click the UPDATE button after that .
Then start clapping because you did it.