If I understand your post correctly, you want to stream music from your PC to an Echo Dot? On My Mac, if I open Spotify for example, and begin playing a song, at the bottom of the Spotify Window between the scrubber bar and the volume bar are several icons. The one that looks like a speaker on top of a screen lets you connect to a device of your choosing. On the other hand you could simply ask your Dot "Alexa, play ABC by XYX " assuming you've set a default music player in the Alexa app.