The installation process is simple.
Drag and drop the octane.xsiaddon in the Softimage window.

Alternative you can use the File -> Add-On -> Install command in Softimage.
Browse to the octane.xsiaddon and click install.

This plugin contains also a custom toolbar.
To move the button from this toolbar to an other, simple rightclick on the toolbar, choose "Customize Toolbar.." and drag and drop the button.