You can load networks to NDEx in several different ways and to do this you must have an NDEx account. The simplest way is through Cytoscape, if you can open your network in Cytoscape, you can easily save it to NDEx. Alternatively, you can load networks to NDEx using your custom program/script. Finally, you can load networks to NDEx via the web user interface if they are in CX format. Please refer to THIS GUIDE
for additional information.