Where are instructions to use this module

The Module does come with a README, which is usually a place to start.

However, if you can't find the the configuration page for images at all, then your question seems not to be about Imagecache Actions, but about working with image styles in general.
In Drupal 7, you should probably start with the documentation page : Working with images (Drupal 7). That will probably tell you that you need to visit Configuration > Media > Image Styles.

Can I ask you, is it so hard to look for the instructions first before complaining that there aren't any?

dman,
I would agree that documentation should be improved.

help file "readme" for d7 version for both dev and 7.0.0 versions is almost empty.

For me it has been also around an hour that I'm trying to figure out how all this stuff works.

For example, I added image style for watermark but still can't link it to CCK image field.

I found that I should change display settings for image field choosing one of available image styles.

Please create some documentation for D7 version.

How to link image styles with CCK fields is functionality provided by Drupal core in D7, and is documented well in the link I gave above.
Imagecache Actions does not provide the UI you are looking for, it just adds some more options to the system that already exists.

If you can suggest documentation for working with images and custom effects that is better than already exists at http://drupal.org/documentation/modules/image - please edit that handbook page directly

dman,
thank you for clarifications.
Could you please just mention on project's homepage and/or in readme file, the same information you gave to me and fender, smth like:
Documentation

Imagecache Actions does not provide new UI, it just adds some more options to the system that already exists.

How to link image styles with CCK fields is functionality provided by Drupal core in D7, and is documented well in http://drupal.org/documentation/modules/image

Fair enough :-)
There are indeed a number of interlocking modules and settings that combine to get the 'watermark' (or any) effects, so a short roadmap can fit into the docs as you suggest.

thank you man))

Documentation is not yet perfect but the README (in the latest dev) now does contain a fair amount of information. Documenting the separate effects and their parameters still needs a lot of work, but we don't need this issue to stay open for that.