You can use the Quick Profile button  on the main toolbar

Clicking this button will open a search box.

Enter the ID of the person or List you are looking for and click Find profile.

This will display a user or List profile panel, from where you can choose to follow or unfollow as normal.

NOTE: This does NOT create a search column, it is just a one-time lookup for a Twitter profile

You can also click on a username or Twitter List name anywhere you see one.

This will open the profile for the user or List, from where you can then Follow if you wish.

You can also follow or unfollow someone directly by using Other Actions > User > Follow on a tweet by that person.