WARNING Not functional yet since slic3r's return codes are behaving unexpectedly. WIP for others to take a look at and maybe build upon.
Install the plugin like you would install any regular Python package from source:
pip install https://github.com/OctoPrint/OctoPrint-Slic3r/archive/master.zip
Make sure you use the same Python environment that you installed OctoPrint under, otherwise the plugin won't be able to satisfy its dependencies.
Restart OctoPrint. octoprint.log should show you that the plugin was successfully found and loaded:
2014-10-29 12:29:21,500 - octoprint.plugin.core - INFO - Loading plugins from ... and installed plugin packages...
2014-10-29 12:29:21,611 - octoprint.plugin.core - INFO - Found 2 plugin(s): Slic3r (0.1.0), Discovery (0.1)
TODO