Assuming the gym you visited is on beyond the whiteboard, and their WODs are public, you can 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, and click the "Post Result" link to post your workout result. If the gym you visited is not a member of beyond the whiteboard, you will need to create the workout before you log a result for it.