Assuming the gym you visited is on btwb and their WODs are public, follow these steps:
- Find the Gym you visited in the Gyms List
- Find the WOD you did in their activity feed
- Click on its name to visit that WOD's page
- 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.