Yesterday I subscribed to wpmudev, but now I have problems.

Hello.
Yesterday I subscribed to wpmudev, but now I have problems.
First of all, I have to explain that I am from Spain, and my English is not too good. Besides, I am not an expert on WP. So it seems I didn´t understand correctly what wpmudev was
So, I have a few webs with WP, and I didn´t know the difference between WP and WPmu. So I thought your plugins would be good to my webs. But they aren´t.
So, I have installed and activated the plugin “Invite”, and now my web (www.valoracionesyreformasdeinmuebles.com) is absolutely blocked, it just shows a text which says “The Invite plugin is only compatible with WordPress Multisite.”. Could you tell me how to solve this problem?
Besides, given that your plugins are not useful to me, should it be possible to refund me the money I paid?
Thank you,
Jose Luis Hernández