SecureSocial uses an Ivy-Style repository, not Maven style. Hence, you have to declare your resolver to use Ivy style:
resolvers += Resolver.url("sbt-plugin-releases",
new URL("http://repo.scala-sbt.org/scalasbt/sbt-plugin-releases/"))
(Resolver.ivyStylePatterns)
(line breaks added for readability)
Maven and Ivy dictate different directory structures. SBT supports both, but defaults to Maven style. So if you want to use a repository that uses another directory structure, you have to tell sbt which structure that is.
Furthermore,
addSbtPlugin("securesocial" %% "securesocial" % "2.1.2")
is not the right choice to include dependencies - it is used to include plugins for sbt itself. Instead, use
libraryDependencies += "securesocial" %% "securesocial" % "2.1.2"