The help note tells me to use the "Activity Settings Page" but have no idea how to find it.
To change the max grade, click on the activity, then click on 'Edit settings' in the 'Administration' block. In the 'Grade' section, you can then choose what to grade the activity out of (this is standard across all gradable activities in Moodle).
I don't think there is any way to set the minimum grade for an activity - the closest you could get would be to visit the gradebook (Administration block > Course administration > Grades) then select 'Categories and items > Simple view' from the drop-down at the top of the page. Once you are there, you can click on the 'Edit' icon for a particular grade, click on 'Show more ... ' (or 'Show advanced' if you are using an earlier version of Moodle) and then specify an 'Offset' to be added to all student's grades (you will also need to reduce the maximum to make sure that the grades do not go above that).
My idea is to start the student point value to -10 in the forum making them participate or lose points. I went to the grading section and set a student grade to -10 but it tells me that I cannot set it to below the minimum.
From your suggestion, I do not see a direct method to change the Maximum as a setting. Is this done my selecting a scale set up under Grades? I tried that. When I grade a comment in the Forum, for example. my scale goes from -10 to +10 but it shows a min/max range of 0 to 21. I doubt I would be having problems here as well as elsewhere if the scale was based on value rather than on ordinate position or index.