When the policy scope is set to Specific Computers, you should see an "+ Add" button on the far right. Click that and on the view that appears there will be a series of tabs. One of them is Computer Groups. Click that and then you can pick your Smart Computer Group or groups to add. You should see "Add" buttons on the far right of each row for the corresponding group. Click those to add them in and when done, click "Done". You can use the search field or just scroll the list until you find the group you want to add.

if for some reason you don't see any of this, you'll need to check permissions to make sure your account isn't restricted in some way, though I don't think there's a way to enter Edit mode for a policy and then not be able to change the Scope.

Thanks so much. I'm not sure how I missed that.