25-04-2013 13:00 - edited 25-04-2013 13:02
Only xap files from the store can be installed on the 520. Save them on the memory card and they will be installed from there. Xap files from alternative sources cannot be installed on standard devices.
See this link: http://www.windowsphone.com/en-gb/How-to/wp8/apps/
You need to ask Microsoft why they have chosen not to add a file manager to WP8. Personally I have no need for one as individual apps manage files themselves.
You can ask Microsoft here: http://answers.microsoft.com/en-us/winphone