How do you increase buffer size?

Is your computer a laptop by chance?

If yes, stop by your local computer tech shop or take it into work and ask the IT guy to show you how to tweak the buffer settings.

I would look in the set up software. For example in JR Media Center 17 you go to:
"Tools">>>"Options">>>"Audio">>>"Output">>>"Settings".

I set my buffer to 0.6 ms (micro seconds) with a 10 second head start from when I push play. I also cache everything from 6GB of memory. Look for something similar in your software. OR...

Invite the local computer geek over for a beer or pizza.
Good Luck!

One person? Really? I can't believe this question has been posted for one full week and only one person responded. Realremo was kind enough to reply (thank you R.), but I still don't have a clear, step-by-step answer to this. It's amazing, for as many people who have commented on increasing buffer size in other threads I'm shocked to see such a lack of answers.

well, that's good, happy listening.

Since I got the A2 to work by changing the PureMusic menu, I haven't heard any "clicks".

As I understand it, the buffer is the amount of memory a software player reserves to stream music into RAM, then into the USB converter. For you this would be a setting in iTunes or Pure Music. I'm a PC using JPLAY, and the current beta has different buffer settings for different sampling frequencies. JRiver also has settings for prebuffering, which delays playback for a few seconds while music is loaded into memory.
There are other things that could cause the pops/clicks, like too many processes going on your Mac mini, but from the looks of your system it's a dedicated machine, so that's probably not it. Also, not having enough RAM can be a culprit.
Since you got the A2 going, do you still hear the clicks?

Anyone?

Also, I got the A2 to work by choosing it in PureMusic's menu.