In the browser, the insert synth page should like the insert plug in pages.  In other words, a page with folders, rather than the drop-down menu at the "+" sign.  

it's right there in the browser (although whether the labelling is consistent may be debatable)
 
if you go to "Plugins" you see 3 tabs: Audio FX, MIDI FX, Instruments
 
@ "Instruments" you find the requested folder like structure, fully customizable with plugin layout - drag and drop from there will insert new synths ...