You add/link subtree by clicking on the SUBTREES->Add/Link Subtree. Enter details of the remote repo and local path.

Expected:
I should see the Subtree in the UI and manage the Subtree using UI.

Actual:
No Subtree is shown but files are remote subtree source code is merged into current project and available on harddisk.

Also some what related... on the dialogue for entering Subtree details, last field "Local Relative Path:" but when you choose a path it gets absolute path. While the Submodule dialogue does this correct.