The 'Manage Variables' option is not available or is inaccessible in the HTML5 version of flowgear. https://developers.flowgear.net/kb/Node:Variable_Bar
^ It says to right click the Node Header and choose 'Manage Variables' but right clicking the Node does not do anything. The only option available when clicking the gear icon on then node is the option to delete it and the option to check help which links to the page above.
In the new console, click the plus icon below the properties to add properties. Not all features of variable management are migrated to the new console yet so you may want to use the legacy one if you need to set variable types.