Do you mean CS5.5 and CS5.0? Export to .idml in CS5.5. Current versions fo CS5 and CS5.5, respectively, are 7.0.4 and 7.5.1.
There are no versions 5.1 or 5.5 Version 5 is CS3, and the last patch was 5.0.4
if you want to downsave an InDesign CS5.5 file (recent version number is 7.5.1) to CS5 (recent version number is 7.0.4) use "Export…/InDesign Markup(IDML)" and open the IDML file in InDesign CS5.