You can access them through the Req object. The code below will iterate through all the values and you can do what you need to with it.
for(s <- S.request; r <- s.params) {
val (paramName:String, paramVals:List[String]) = r
}
If you just want the paramater names, you can use s.paramNames
instead of s.params
Full api doc here: http://scala-tools.org/mvnsites/liftweb/lift-webkit/scaladocs/net/liftweb/http/Req.html