There have been many changes with the MashUp addon recently through the MASH2k Repo due to their advertisements and popups. I will provide the new instructions on how to re add it to your system to get things working again, however, I would be weary of future MashUp advertisements and popups that may disrupt your system.
This is A LOT like installing the fusion installer which I have posted a video blog about before.
So rather than me posting a new video, please use the one above as a reference and the settings below to.
Source : http://install.mashupxbmc.com
1) From the file manager add the source above.
2) Browse to Addons under settings then choose Install from Zip File
3) Browse to the source you added (which would be whatever name you called it) such as Mash2K or MashUp
4) Install the only repo under the repo folder
5) Browse to Addons and this time choose ‘Get Addons’
6) The Repo you just installed should be at the top ‘All addons by Mash2k’
7) Under the Video section of his repo is the MASHUP Addon. Once installed it should be with all your other video addons.