The add/link subtree dialog allows browsing for "Local Relative Path".
Upon selecting a path, it is placed into the field with a full, absolute path (see below).
e.g. /Users/david/Documents/Programming/pantheon/phw_pantheon/sites/all/modules/quiz

This causes the add/link to fail. Manually removing the leading part to leave only the relative path from the docroot allows the command to complete correctly (linking an existing subtree in my case).
from example above:
sites/all/modules/quiz
works fine.

I wonder if this also causes the initial Add to fail.

Thanks!