Hi KingMi21,
Just pass "Licensor" not "plus:Licensor". "plus" will be prefix for "http://ns.useplus.org/ldf/xmp/1.0/", you can verify from namespace. From you code, it seems you are just trying to get the value of Licensor. you can check whether Licensor is array or simple and based on that you can use either getProperty or getArrayItem.
-Sunil