Okay let's attack this step-by-step.
First get your files together.
Download the Heimdall 1.3.1 application for OS X as explained here:
How to Use Heimdall on Mac OSX to Install Kernels and ROMs! - YouTube
Don't worry about procedure to flash just yet, we'll get to that later. Right now we're just getting the files you need.
The Kernel I used to root my 2.3.6 i777 was this one:
GalaxyS2ATTRootZedomaxMacLinux.zip
The link to download that is on that page. Again, don't worry about procedure, just go get the file.
Finally, you mentioned ShoStock2. I'm giving that a spin now and it's a nice package because it includes the cherrypicker kernel, the gapps files as well as the ICS rom, all in one package. It's pretty much a one stop shop for ICS on the i777. You can get the package here:
SHOStock ICS ROM for AT&T Galaxy S2 SGH-i777! | Galaxy S2 Root
For the Zedomax kernel and the ShoStock rom just save the files as is, don't expand them. Once you've got those saved and heimdall installed, let me know and we can move on from there.
I will be in and out today so i hope you're not in that much of a hurry.