How to install .rbz extension in Sketchup 6.0 version
Rename the RBZ extension to ZIP and unzip the file. Install the content of the resulting folder as normal. Note that some of the latest extensions may not work in SU6.
The general official guide is here:
http://help.sketchup.com/en/article/38583
I wrote a more detailed guide some years ago:
But not that it might not work in SU6. A lot have changed and been fixed since that old version. More and more extensions require features from newer versions.