Hi,
set up my machine at work (WXP 32bit) and MercurialEclipse in Eclipse Helios from market place worked without problems.
At home I did the same procedure but on W7Pro 64bit. MercurialEclipse itself got installed, but it errored out upon Workspace restart that the hg command was not found; and it's true, there was none installed.
Comparing the plugin configurations within both Eclipse installations I discovered that W7Pro 64bit misses the Windows Binaries for Mercurial plugin. Is that architecture dependent? Is there a way to fix this?
Update:
I verified with multiple installations: MercurialEclipse requires an external binary to properly work. I was confused because through testing on my first installation I also installed the Windows Binaries for Mercurial "somehow". On my second machine I didn't and I also hadn't installed MercurialHg.
By default MercurialEclipse simple requires hg in its configuration which implicitly assumes the binary must be available in the path somewhere. The Windows Binaries for Mercurial works a bit different, it installs a private copy of hg.exe inside the eclipse/ folder somewhere and sets the absolute path in the MercurialEclipse configuration. Both ways work equally (ignoring differences which could be arise due version differences), since either installation would use your mercurial.ini from your home directory.