Sounds to me like you are trying to work with CS5.5, CS6 or CC files (that's the standard, stupid, totally wrong and useless error message you'd get in CS5). There are no plugin upgrades, presuming you've actually updated the program to verion 7.0.4, and you cannot open newer .indd files in CS5. As always, ID open files only from that version or earlier.
There is no longer any upgrade pricing available for a CS6 perpetual license (and it won't open CC files). You could consider subscribing to Creative Cloud, which will give you access to both CS6 and CC, and I beleive there is a first-year discount avaialble still for registered CS5 users.
For CS5 owners there is upgrade pricing available for CS6.
I stand corrected. I can't keep up with what is and isn't being offered any more.