It should show up once you’ve authorized IFTTT to access it through ST. You have to authorize each individual device, so if you add any new devices, you have to go through the authorization process again. You can start this from either the IFTTT side or in the ST mobile app.
In the ST mobile app, go to the Dashboard and tap on the “SmartThings Internal” category.
Then click on the tiny gear icon on the IFTTT tile to open up its details page:
This will allow you to specify the devices for IFTTT control. Click DONE at the top right when finished.
After this, the devices should show up as selectable from IFTTT.
That said, what exactly is it you want to do? ST is actually somewhat unusual in allowing IFTTT access to locks–most IFTTT channels don’t. So for example Harmony will not allow IFTTT control of any activity that includes locks, even ST-controlled locks.