<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
launchctl load ~/Library/LaunchAgents/watch.and.lol.plist or relogin to load the agent.
There's a launchd gotcha « Managing OS X though:
It turns out that if a file is listed as a launchd job WatchPath, it must exist at all times, or launchd removes the path from its list of paths to watch. So you can't watch for the creation of a certain file; you can only watch for changes to the file.