search @username@pixelfed.social and you should see the user pop up.

@user@server.url just like any other account. It should show up. Another option is to search for the post on the top of lemmy and then navigate to their account.

Example: @dansup@pixelfed.social

If that doesnt work, you can try this: https://lemmy.world/u/@dansup@pixelfed.social

in other words your telling lemmy to go to his personal account, which happens to be on another server.