I wanted to ask: if Is it possible to use allready-made assets and whether will they be counted as criteria "Use of Houdini". For example: an open asset "tree generation" https://www.sidefx.com/contentlibrary/tree-generator /.

If not, then will all assets must be created only in time of the "Game Jam", or can ill use mine own blanks?

I'm asking this because I didn't find a clearly answer in the rules 

Yes, you can use pre-made Houdini assets that will be counted as "Use of Houdini". You will need to list which pre-made assets you used and which assets you built yourself during the game jam.