if you have a fresh install of the sdk it might come from the jdk/bin not being set as executable.
(this is only true for the linux version)
But not sure if it's you're issue there.
Before doing anything, check if you can install other plugins, if not close the sdk and try :
Go to => /home/myName/jmonkeyplatform/jdk/jre/bin -> check if all files there are set to executable. if not, then set them as executable. -> run the sdk and try to get some plugins.
ps : i don't remember well the path of the folder, be kind .