Hi Roman,
My Help&Manual version is 6.5.5 Build 3020.
That is out of date. Please always check that your copy of Help & Manual is up to date before posting questions or writing to support. You can download the installers for non-current versions of Help & Manual here:
http://www.helpandmanual.com/downloads.html#oldver
The /decompress reference needs to be removed from the help. That has actually not been supported since Help & Manual 4, when XML was still an output format and not the native storage format for all projects. It is just an oversight that that is still included in the documentation.
Since Help & Manual 5 the original project format is already XML. The HMXZ project contains exactly the same files as the HMXP project. No conversion is required. The HMXZ file is just a zip archive, which works in exactly the same way as DOCX files, which are also normal zips. To convert HMXZ to HMXP you just need to change its extension to .zip and then unpack it with any zip utility. The main project file inside the HMXZ is always called "project.hmxp". It is generally best to change that to the name used for the HMXZ file. When converting back you need to remember to rename it back to "project.hmxp", otherwise it won't work.
