The latest version of the Camera Raw plug-in available for Photoshop Elements 7.0 is Camera Raw 5.6. The latest ACR for CS3 is even earlier. Neither of them include the 550D in their list of compatible Raw types and - even worse - the 550D is not covered even by the latest ACR, version 6.2. (The Camera Raw 6.2 plug-in is not compatible with versions of Photoshop earlier than Photoshop CS5, or versions of Photoshop Elements earlier than Photoshop Elements 8.0)
Obviously, the 550D will be added to ACR very soon but it won't help you because your software will still lag behind.
So you're left with a couple of options.
1. Upgrade your software to CS5 or PSE8 (expensive)
2. Use the Canon Digital Photo Pro software that comes with each new camera (but I don't think it's available online so if you don't have one you'll need to borrow a recent version from somebody)
3. Convert your Raws to DNG - the
DNG Converter version 5.7 included the 550D. (This is a little odd, as the list of cameras supported by ACR6.2 does not include the 550D - maybe a mistake by Adobe there). If you download the DNG converter it may come with the latest ACR - which you could certainly try to install but probably will not work)