The conventional method of following another user on Tumblr requires you to log in to your Tumblr account, visit the user's blog and then click the "Follow" button in the overlay menu of Tumblr options. However, if you can't see the Follow icon on a user's blog, or if you have only the user's email address, you can still manually follow any Tumblr user by visiting the Following page.

Manually Following a User

To follow someone without clicking "Follow" on the user's blog, log in to your Tumblr account and click the "Following" tab in the Account section of your dashboard. This tab lists the number of blogs your main Tumblr account follows. Enter the username or email address of the person you want to follow in the blank field at the top of the page, and then click the "Follow" button next to the field. Tumblr automatically adds the user's blog to your following list.