Just just "caf" to enable or disable caffeine. There are others with more functionality (i.e. activate for a number of minutes) if you want those.
There are more sophisticated Caffeine Workflows available for Alfred2, but some people like the simplicity, so here is this one. However, if you're using 10.8+, then I'd recommend uninstalling Caffeine in favor of using the native Caffeinate utility. You can then use my Caffeinate Workflow.