How to get a digital signal out of my iMac's USB?

What do I need? or does it just happen when I plug it into a DAC? I am a bit confused.
Use something like a Hagerman HagUsb.

Think of USB as a kind of highway. Same thing with Ethernet and WiFi. All of these are media agnostic - like Route 66, they don't really care what kind of data (file type) the trucks are carrying.

Think of your music file as the load on one of the trucks going down the highway. The exact design of the truck will depend on where it will end up, which is your DAC/garage.

SO - through your computer or remote controller, you access your hard drive (call is the term) and select a particular file - ie go gas up U2>Joshua Tree> Streets Have No Name.

Dispatch the driver to take the Ethernet or USB or WiFi highway from the computer to the distribution center. (Or to a thousand if that's what you want)

Check the file in, one palette (packet) at a time. Then, move the file from the distribution center to the assembly line using the Toslink, I2S or SPDIF forklift.

Add water, pound down a beah and chill =)

ie your computer calls up the file. it moves across your connection scheme to the DAC where it is decoded and sent on to your preamp. if you understand what happens after that you are a genius