Ugh... Your tale sounds all too familiar. I had a very similar situation on my previous MBP. Tried to install some stuff (MAMP was a cohort in the chaos here too...) and ended up spending three days on the command line pretty much at that stage running commands that I copied and lasted from blogs and forums where people had posted after having similar issues but not really knowing from shit what those commands actually did.

Ended up backing away slowly and never looking at RoR again.

Honestly: I am an experienced programmer and all I could make of the install process (plus the attempted installation of some gems) was an utter dogs breakfast.

So as anecdotal evidence goes - add mine to the list please.