That error is indicative of trying to open a file created in a later version of InDesign. Have whomever sent it to you save it back via IDML and don't expect it to open perfectly.
Text will likely reflow and any new features will be lost or badly mangled.
Yes, I have figured that out. I am confused though...because this other file is created in CS5 as well....When I do the .IDML i loose all of the text.
What makes you so sure it was created in CS5? Hint: A get info will only give you the latest version you have installed, not the actual version that created it.
You don’t lose the text, but due to differences in text engines between version reflow is good possibility.