Are you right clicking (on the ground, or somewhere) to activate the spell, and then left clicking on the item? Try getting closer, if you can, or increase the skill level (Telekinesis spell books can be bought).
Telekinesis triggers the default action of the item. For things like the teleporter stones (default action is to use), letters (read) and chests (open/close) your character will run over to them even if you are in range to use Telekinesis. Try practicing with dropped gold or equipment, to get a feel for the range of the spell.
If you have both teleporter stones with you, you can toss a teleporter into the sewer beside the item, drop the other on the ground and use it, pick up the item (you will not be able to walk in the sewer) and drag-and-drop the teleporter stone into your inventory, then use the teleporter to get out of the sewer.
