$ grape resolve org.codehaus.groovy.modules.http-builder http-builder 0.5.0-RC3
  # will tell you where grape is looking and give detailed config information
  
or:

$ grape install org.codehaus.groovy.modules.http-builder http-builder 0.5.0-RC3
  # will attempt to download the JAR and transitive dependencies

Works out for me.. thought 0.5.0-RC2 is just fine.

Thanks,

Roger