That might be the problem. You should try to upload it via the wordpress plugin uploader. I’ve attached a screenshot where you can find it. If you upload the file named ajax-search-pro.zip which is inside the archive you downloaded from codecanyon.
If it’s still not working, then can you provide temporary administrator access to your website, so I can try to solve it manually? (you can edit your first post in this thread and there you can fill out the private fields, only you and me will be able to see the usearname and password).