I am not sure you want your NAS to be the master browser; usually it is a PC that does that. I have read that some custom firmware for my ASUS router can set the router as the master browser, and that makes some sense to me.
It appears you may not have shared the device on your network, or used the WD Discovery program. If not, AND/OR you do not have the complete User Guide for both units, download them from the WD support area and dig into them.
Have you enabled Windows File service in the control panel of your DS413 and added it to your Windows Workgroup. You should install their Media Server program also.