(Note: You can find this code snippet on Repost.Us. Simply log into your account and in the Repost.Us Installation of your Site Summary it is the second code snippet. This tutorial is done with the free Redux template, some templates may vary on placement)
Step 6: In your Tumblr template's HTML, locate the "block:text" (you can do this easily by simultaneously holding down the Ctrl and F keys on Windows or the Cmd and F keys on Mac) and paste the button code located on the Repost.Us installation page (the first box in the previous screenshot).
Step 7: Save your results, and you're done! (Click Update Preview >> Appearance >> Save)
If you have any additional questions about how to get Repost.Us working on Tumblr, please feel free to contact us!