Using Netbeans 6.9.
I wanted to download the SOAPUI plugin from soapUI NetBeans Plugin.
Although the description says "com-eviware-soapui-netbeans-module.nbm", when you actually download the file, you'll find it's a zip file.
Netbeans doesn't like loading zip files. A little research at Mr. Google shows that a .nbm file is essentially just a zip file with a different extension.
So I simply simply renamed
com-eviware-soapui-netbeans-module.zip to com-eviware-soapui-netbeans-module.nbm
and the plugin installed no problems at all.
It does warn you that the plugin is not signed but just ignore that and continue.