I`m just trying to figuer things out here...I have loaded Jessie with Bra and Jeans. If I go to the mat.-room to change the materials on lets say the bra there are no materials to choose from in my Library, mostly I find Hair and other stuff I do not know. (Why hair in the material-room anyway?) But no materials. Do I have to download materials or should there be som mat. to chose from in Poser7 itself? Is it just me, that I can not find what I am looking for?

The material room could be called "material editor". That is, there are some material presets around, but (at least for me), I prefer to play around with all the nodes and stuff and make my own materials.
I think there are indeed no "stock" materials for the bra and jeans. But try and play around with the nodes, you'll be surprised at how many things you can do just tinkering around.

The library window has a button "Materials". You have to click that. This Library button is only visible while you are in the Material room.

I don't know if there are actual materials for those items. Quite often, people put materials into "Pose" files instead. These are called "Mat Pose" files. (They are a kludge. For a long time it was the only way to put a "set" of materials into the library as a one-click solution.) A lot of the default content is in the library as poses instead of materials.

Nowadays, we have "Material Collections". But a lot of people are intimidated/confused by the Material room. Because the programmers made the unwise choice to only show the materials library if you are in the material room, and that the vast majority of Poser users who are afraid of the material room, content providers tend to continue publishing material collections as Mat Pose files instead of as materials.

This general theme is one of the most amusing aspects of Poser in my opinion.

"Where are the hairs?" answer: props
"What about XYZ hair - I can't find it?" answer: oh, that one is in figures
"Where are the materials?" answer: poses
"Are custom characters in Characters?" answer: no, poses

Quote - The library window has a button "Materials". You have to click that. This Library button is only visible while you are in the Material room.

Thank you for your answers. And Yes, I did click the Mat.tab in the Library. Thats where I didnt find any materials, so to speak. But anyway, nodes and stuff are things I need to explore. Do you know any good tutorials for mat.room and nodes and these things. I´m eager to learn..

Check out this thread:

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2722867

The links at the top of the list are "newbie" tutorials that explain the make up of the material room.

The other links in that thread are tutorials and discussions that explain how to do all kinds of things in the material room.