How can I sync a folder *to* android using syncthing?
Here's what I did. 

Firstly I was in the same situation - devices were visible to each other, windows had the files, android did not.

I created a folder on android with syncthing and gave it the same name as my folder on the PC ( not essential ) but crucially, I used the same folder ID as the folder on the server.

Then I selected the server ( slide switch on ) and saved my folder in android. Now they are syncing files.

Summary make sure your folders have the same folder ID (on the general tab).

Hope this helps. Please give it approved answer if it solves.