Ouch!
You can still do it through the IDE, but it’s not intuitive.
- click on “community” at the top right of this page.
Two) click on “developer tools” at the top right of the first page of the community section.
Three) sign in with your SmartThings account
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choose the “my locations” tab
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on the list of your locations, choose the one where you want to add a new mode. This will open its location detail page.
- scroll down until you see the list of your existing modes. It’s not obvious, but there is a clickable link called “create new” at the top of that list. Click on it.
- this will open a page where you will be able to add a new mode.
@slagle if there is some new way to do this through the SmartThings mobile app that you guys just haven’t told us about yet, please let us know. 
(Also, if there is a shard issue on this so that clicking on developer tools takes some community members to a page where they can sign in but they can’t actually see their own locations, I give up. 
SmartThings staff can explain it for those who can’t find their own modes page.)