The crash on opening Preferences is an unresolved regression with OS X ActiveTcl 8.5.12.1, the current ActiveTcl Community Edition, that causes Tk to crash when selecting the IDLE Preferences.. command.  The regression was also in ActiveState 8.5.12 but not in 8.5.11.1.  The crash appears to be related to the Tk crash-on-copy problem documented in Issue15574.

Until the problem is resolved, if you have access to ActiveTcl 8.5.11.1 (ActiveState only makes the most recent version available to Community Edition users), you can install that.  Otherwise, you could either temporarily "deactivate" the ActiveState Tk by renaming it, for example, after quitting IDLE:

sudo mv /Library/Frameworks/Tk.framework /Library/Frameworks/Tk_disabled

This will revert to the Apple-supplied system Tk, which has some critical bugs in 10.8 Mountain Lion (it's totally unusable for IDLE in 10.6) but might be OK for your usage or, at least, for updating Preferences.

To "reactivate" ActiveTk, rename it back:

sudo mv /Library/Frameworks/Tk_disabled /Library/Frameworks/Tk.framework

Once you've made some preferences changes, it is also possible to manually edit the configuration files created in ~/.idlerc.

I will update this issue with a cross-reference to an Tcl/Tk issue when available.