CC5.5 should be able to open an IDML file.
Check with the person who sent it to you, and confirm that they made the IDML by using Export, and not by just changing the extension.
There is no plug in. It should just work.
What is the error message you’re getting?
When I try to open it it tells me it don't support that kind of file.But a couple of days ago when she exported the file to me and was still useing Creative Cloud I was able to open it in my CS5.5 InDesign program but since I injstalled my program on her computer and she canceled Creative Cloud now I can't open it and I can't get any help from Adobe because I'm useing an old program and they have no support for that so it looks like I'm going to have to download a free trial verson of Crteative Cloud if I want to open that file.