Check youtube for synology set-up guides, if you don't want to read Synology's quick set-up guide.

I believe you would need to use File Station to configure the shared folder.

Then, use the 'network path' for that folder.

Re-reading your original post, I think you have that much done.

When configuring my music server to retrieve files from my NAS, I had a tough time, but don't recall specifics. A quick synology forum search found something that looks familiar though:

DSM Control Panel / Firewall / Ports - Select / check the box for "Windows File Server"

It was something as simple as checking the right box, if I recall.

I found my answer searching in the Synology forums.