You need to set bit 6 of "low_fuses".
The very easiest way to do this (IHMO) with the Arduino environment is to edit your boards.txt file (search for it in your arduino installation) and find the section for the board you're interested in.
It will look something like this:
pro328.name=Arduino Pro or Pro Mini (3.3V, 8 MHz) w/ ATmega328
Make a copy of that section, change all the "pro328" to something like "pro328ck" and change "pro328ck.name" to something like "Arduino Pro Mini (3.3V, 8 MHz, CKOUT)" (That's what you see in the 'boards' menu).
Now you can edit the "pro328ck.low_fuses" entry to whatever value you want. If you select that entry in Tools->Board and do a "Burn bootloader" the fuses will be set. nb. You probably need to restart the Arduino IDE every time you edit that file.
The clock output bit is bit 6 of low_fuses, you need to set it to 0 without changing any other bits in low_fuses.
If you do it correctly the clock signal should appear on digital Pin 8.