First of all create a new plugin page on http://www.freedomotic.com/node/add/plugin (requires a registration). After your plugin page is created enter in Edit mode and look at its address in your browser bar. Take note of the number in it for example if you have www.freedomotic.com/node/850/edit your nodeid is 850, remember it.
Open your pom.xml file and write this number as the value of nodeid property. This way you are linking your plugin with the related marketplace page on the website.
Then to upload just do (from command line)
cd FREEDOMOTIC_ROOT/plugins/devices/YOUR_PLUGIN_NAME mvn clean deploy -P market -D username="YOUR_FREEDOMOTIC.COM_USERNAME" -D password="YOUR_FREEDOMOTIC.COM_PASSWORD"
Redo it any time you want to update the downloadable binaries on the marketplace page.
Please follow the instructions at http://www.freedomotic.com/wiki/external-developers-workflow-recommended