After looking at the source code of this plugin, I found out I had to specify the findbugs classpath to make the sbt action work. For those who run into the same problem, here is the solution.
findbugsClasspath := IO.listFiles(file(<your findbugs lib location>)).filter(_.getName.endsWith("jar")).toSeq.classpath
This should've been pointed out clearly in the wiki.
BTW - I believe this could also solve the similar issue by the cpd sbt plugin.