I'm having problems with this addon also. I have done what the instructions have stated. Put the mediarss.php in the addons folder and then add <MEDIARSS_AUTODETECT_LINK> inside the browse and image template headers.
When i view the page in the brower it doesnt work. After viewing the page source it shows that <MEDIARSS_AUTODETECT_LINK> doesnt render to what it's supposed to. it just leaves it alone.
Any ideas what i did wrong?