If you open a .tm file which is under version control, i.e. in a local working copy of an SVN repository, then the items in the version menu should not be greyed out.
First: what TeXmacs version are you using?
Second: The svn "plugin" checks for the presence of an .svn directory anywhere in the parent directories of the document you are viewing. The menu will only be greyed out if TeXmacs fails to find one. This could be either because of:
* the directory not being there: is your file truly in an SVN working copy?
* some directory not being readable: are permissions for all parent folders set to something that allows TeXmacs to read their contents?
* bogus caching. This is more than unlikely since there's no hard caching of this particular information, but had you opened the file with TeXmacs before it was versioned? If yes, try adding a new file to the repository then opening it with TeXmacs.
Best,