Can I add a job in search mode?
No you can only add Group ID or panda links. Search mode is not always the best way to collect hits. It's a slow process because it has to do a search first, find the hits, send a panda request to mturk and then hope to get the hit. The way mturk works when a new hit gets posted is slow too. panda links will find hits even before it gets to a search page so always try to use a panda job.
If you really need to add a search job then the easiest way is find a hit from the requester using the helper script or search mturk button. Use the add button and then change the action mode to a search from a panda. It should already have the Requester ID. If you can't find any hits from this requester then just and any panda and fill in the requester ID. Change it to a search mode and it should work.