The most efficient way to mix and audition IRs is in a DAW. There's a post here and a sample Reaper project here that describes the process:
viewtopic.php?f=15&t=12062&start=10
If you want to use the mixIR, the easiest way to get started is to get a link to the mixIR by going here:
http://www.redwirez.com/getmixirlink.jsp
It will email you a personalized link to the mixIR app.
It works just like the offline version (which is to say, "just barely"). You have to paste a recipe. The recipe is text. Trying to copy and paste the files themselves won't do anything. People have been posting recipes on the forum for a little while. One of the preferred ratios seems to be 74% R121 Cap 1-2", 24% SM57 CapEdge 0", and 2% KM84 Back of Cab. For the online mixer you can just paste the file name of the IR, followed by its percentage in the mix like this:
Marshall1960A-G12Ms-R121-Cap-2in.wav,0.74
Marshall1960A-G12Ms-SM57-CapEdge-0in.wav,0.24
Marshall1960A-G12Ms-KM84-BackCab-0in.wav,0.02
The percentages should add up to 1 or pretty close to it, but not more than 1.
You have to find the IR files on your computer to copy their names. They should be in the folder where you initially unzipped them. If you just downloaded the Axe-Fx sysex files, then you can use those names, but change the .syx to .wav after you paste or type the name in the recipe box.
I can't provide an easier to use tool at the moment. Getting a tool that works on Vista, Windows 7, Tiger, Panther, PPC macs, Intel macs, and all the other variations out there has taken a lot of time away from sampling cabs and processing the stuff we've already sampled, so we're behind schedule. Once the new cabs are released I'll start working on a more user friendly version.
Good luck
