Mashup is a third-party XBMC add-on, it is amazingly popular and until recently required a lot of google searching/attempts to get it installed (at least for me). Mashup should be installable on anything you have XBMC on so not just your Pi but any Linux/Mac/Windows OS that has XBMC installed on it and the process should be the same as this one for the Pi !
This guide will explain how to install Mashup in XBMC, to find out more information about Mashup then check this link here or the one at the bottom of this post. If you are super lazy you can checkout my Pibox Sourceforge page for any flashable images of projects i have done previously as well as this one (should be uploaded in the next hour or so).
With XBMC booted you need to navigate to SYSTEM > FILE MANAGER where you will be greeted by a screen similar to this, click ADD SOURCE on the left side panel :
Click the bar which says <NONE> on the window which opens and enter this address :
In the box below enter something like MASH or MASHUP then click OK :
Your screen should now look similar to this one :
Now return to the main menu and navigate to SYSTEM > SETTINGS and choose the ADD-ONS option from the left hand side :
From here select INSTALL FROM ZIP FILE > MASH > REPOSITORIES > REPOSITORY.MASH2K3-1.5.ZIP (NOTE : Your version of Mashup may be different to this) :
Now select GET ADD-ONS > ALL ADD-ONS BY MASH2K3 > VIDEO ADD-ONS > MASHUP > INSTALL to enable and install the Mashup add-on :
Now return to the main menu and select VIDEOS > ADD-ONS > MASHUP and you should now be able to use the add-on :
Mashup is now installed and ready to use, enjoy !