Hello Pilar,
I'm not aware of any manual specifically written for the VISA Session utility in MAX.
This tool allows you to call VISA functions interactively, without the need to write any code.
What may help you is to check the icon that is on every tab (lower left corner). Every tab performs a function call, and some let you pass parameters/values and/or read/see the results of that function call. The icon shows the equivalent LabVIEW function, and if you have LabVIEW, those icons have help files to better understand their use. I attach a screenshot, where you can see that on the tab, the function is a VISA Write, so I could go to LabVIEW, look at the VISA palette, find the icon and read its help to understand how to use it, the inputs and outputs, etc..
Also, the NI-VISA function reference help that you can find going to (Start->Programs->National etc...) has a list of all the VISA functions in C format, so if you get familiar with those, then the VISA Session is almost the same thing.
Regards,
Jorge M.