If I need a new one how do I know which one to get and where do I connect it?!
The cable coming from the Farida R-16ENA is at instrument level. Your guitar & headphones USB audio interface is perfect for that. No additional equipment is needed.
How do I test the sound card on my computer?
You mentioned recording works with GarageBand. That's great and means jammr should work too.
Set up jammr as follows:
Launch jammr and go to settings (Apple | Preferences…). Make sure the “Input device” and “Output device” are your USB audio interface. Ensure that “Play back my audio” is enabled when using headphones.
If your USB audio interface has a “monitor” knob, turn it off so you only hear audio played by the computer and not your guitar.
Now connect to a jam. Once connected you'll hear yourself and others.
How does the jam work if it isn't actually in real time? Does that mean we aren't actually jamming together, just over one another?
jammr is an interval-based jamming system. That means there is a number of beats, called the “interval”, that everything is synchronized around.
You hear what others played last interval. They hear what you played last interval.
In order to play in sync the interval must match the length of the chord progression. Then it won't matter if you are playing over a D minor that the other musician is playing right now or the D minor they played last interval.
Here is a concrete example. The chord progression is Dm C G. You hear the Dm and play a note that fits. Actually it's the Dm they played last interval but as long as they stick to the chord progression they'll be playin a Dm right now as well. Therefore your note will fit - it will be heard over Dm and not the C or G.
That's how you can play in sync without people around the world without worrying about lag.
The trade-off is you cannot change the chord progression without temporarily causing a “collision” where it might not sound right. So jammr is great for improvization but it's not a tool for rehearsing complete songs from start to finish (they require chord changes). There are exceptions, like playing a 12-bar blues since the progression stays fixed for the whole song.