I got into using DSD+ a couple of months ago - just "playing around" mostly. I have a Uniden 355C (similar to your 350C) and I considered tapping it, but the electrical connections are so tiny that I decided to tap my Radio Shack Pro-96 instead - it has a convenient "tapping pole" popping up from the circuit board. I connected a 10K ohm resister between it and the center connector on a 1/8" miniature mono socket, I grounded the other lug of the mono socket to the heat shield on the circuit board. Then I drilled a hole in the case and mounted the socket onto the case. Voila! I had a tapped scanner!
Not much is said about the actual cable to connect between the scanner tap and the microphone on the computer. This took some experimentation. I have a laptop computer with one socket for microphone and speaker output. It turns out that this needs a four contact plug! I got hold of a Y-splitter for this purpose, and I connected a mono-to-stereo adapter to one end of a 1/8" stereo cable and the other end of the stereo cable (also terminating with a 1/8" plug) I connected to the microphone tail of the Y-adapter. The audio out of the Y-adapter of course can be connected to headphones or an amplified speaker. I plugged the mono-to-stereo adapter into the scanner tap socket. So, now I had my connection to the computer.
Finally, I downloaded and installed DSD+ according to instructions you already have. Hint: You also need to set the audio input of your computer correctly! If you have trouble with this step let me know and I'll walk you through it.
It took a bit of fiddling with DSD+ and the computer to get all of the settings just right, but now I can monitor my local police dispatching on DMR - they are silent no more!
Best of luck with it.