Last time I checked Image Manager doesn’t work with WordPress 2.2 and it’s site still says that it’s only tested with 2.0 and 2.1, so if you have an up-to-date copy of wordpress you won’t be able to run it.
Really though, I wouldn’t bother. ImageManager had an awful interface and created mediocre quality images. overall it was pretty craptastic.
I just use https://pixenate.com or https://www.picnik.com/ , both of which have nice, useful, high-quality online editing servies that are free and let you upload photos, edit them, then download them again to upload to wordpress. (I have no relation or association with those sites, they’re just what I switched to after Image Manager broke and my life improved.)