In the folder where US2 is installed, you'll find another folder called 'Data'. Inside that is appearance.xml.
Open that with a text editor, such as notepad, go to the bottom of the file. Find the last line that says </Object>, place the cursor at the end of it, hit return, then paste everything that's in the appearance.txt file from the asteroid pack. If you're not confident about getting this right, make a copy of appearance.xml first, so you have a backup.
You have a Universe Sandbox 2 folder in My Documents as well. The asteroid_showcase.xml file goes into 'System' folder inside that US2 folder.