I think that plugin is more for when you move your site from somewhere else to jekyll- then you can set it up so that some weird url that tumbler may have used will still find the correct page on a jekyll site.
I think what you want needs to be done at the domain level - you need to do a 301 redirect on the old domain to make everything go to the new domain.
How to do that will likely depend on who does your dns and maybe your hosting.
google is your friend, try googling redirect domain - you want to do a 301 redirect I believe. There are probably multiple ways to do it. Fairly sure you would do it thru dns. I think it can also be done on the old domains website but not sure that is the best way.