Assuming the gym you visited is on btwb and their WODs are public, follow these steps:

  1. Find the Gym you visited in the Gyms List
  2. Find the WOD you did in their activity feed
  3. Click on its name to visit that WOD's page
  4. Click the "Post Result" link to post your workout result.

If the gym you visited is not a member of btwb, you will need to create the workout before you log a result for it.