Your request works for me:
$ curl "
http://closure-compiler.appspot.com/compile" -d "output_format=json&output_info=compiled_code&output_info=warnings&output_info=errors&output_info=statistics&compilation_level=SIMPLE_OPTIMIZATIONS&warning_level=default&output_file_name=default.js&js_code=%2F%2F%20ADD%20YOUR%20CODE%20HERE%0Afunction%20hello(name)%20%7B%0Aalert('Hello%2C%20'%20%2B%20name)%3B%0A%7D%0Ahello('New%20user')%3B"
{"compiledCode":"function hello(a){alert(\"Hello, \"+a)}hello(\"New user\");","statistics":{"originalSize":90,"originalGzipSize":100,"compressedSize":55,"compressedGzipSize":68,"compileTime":0},"outputFilePath":"/code/jsca752286874097e7ef44c9e029737eb83/default.js"}