Crossing platforms is not an issue. The issue is that InDesign CS5 cannot open CS5.5 files. But if you export or save the files from CS5.5 as IDML, you'll be able to open them in CS5.
I too have CS 5.5 and need to transfer files to CS5 machines at school. Yeah, I think Steve is right. I think the message about the plug-ins is incorrect, and may have been removed from updated versions of CS5. I don't think updating plugins, or CS5, will work. As far as I know! Most of the time if you export to InDesign Markup Language, it will work. Sometimes I have found, however, that it does not, for some reason, depending on my computer in the lab at school. But, if you package the indd file, then go back and open the indd from the package, then export it back into the package as an IDML, it will work. Ofcourse, you have to transfer the whole package, instead of just one file, but this has saved my rear-end at school; just open the IDML version from the package (both IDML and INDD will be in the package file).
Yeah, I think Steve is right.
Oh, there's no question. This is a frequently asked question and the answer is easily the same.
I think the message about the plug-ins is incorrect, and may have been removed from updated versions of CS5
No, the flawed message remains in CS5 7.0.4, the latest update.
I don't think updating plugins, or CS5, will work.
Correct, it will not.
Its hard to say what went wrong with your IDML export without more information.