You can use Mio POI Loader.
Download any GPS POI file (Garmin CSV or Navman CSV POI file format) and load it to your Mio. Loading is quite simple and user friendly thanks to the Mio POI Loader installer software. It's available here. Can be used with C210, C220, C230, C250, C320(t), C520(t), C510, H610 and C710. See it's help file for more information.
Once when you install MIO Loader you should have no problems with installation of downloaded speed camera POI files or any other GPS POI files (in e.g. Garmin CSV POI Loader file format).