The update notes to the last version have a section about enabeling wifi in the admin menu. If I remember correctly this will install the needed software for wifi and this has to be done prior to editing the wpa config.
In order to use the web interface you have to start mod-ui first. You can do so by adding a layer with mod-ui in it using the Zynthian interface.
PS: For a command line text editor I would recommend nano - for my taste its a bit more user friendly. (If you are done editing just press ctrl+x and you will be asked if you want to save changes or not)