which leds have you bought (part number)?
the clocks in the link are mk3 clocks aren't they? which bulbs do you wanna replace, just clocks or the ones for lcds?
there are no 12V LEDs... LEDs have forward voltage of around 1.5 ~3V depending on type, so for replacing dial ones you will need resistors... if that is just buy a few different ones (would suggest 5 different values across) from e.g. maplins between 680 and 1k5 or 2k ohms.