Do you mean installing an Operating System image such as Raspbian onto an SD card?
Perhaps following instructions like those published by the Raspberry Pi foundation?
http://www.raspberrypi.org/documentatio ... ges/mac.md
Is it really 'stuck' - or is it just taking longer than you want it too?
The time it takes is entirely due to the size of the image and the bandwidth/throughput of your SD card reader/writer and the SD card itself. You can do the sums for your own setup yourself, but using 'Wintel' hardware running either Windows or linux usually takes between 15 and 20 minutes. Using the NOOBS installer on an RPi (which carries out the same operation 'natively') takes about the same time. In that case there is a progress bar to reassure you that something is happening.
I'd say be patient and give it at least 20 minutes before you conclude it has failed.
If you like feedback, I believe there are 'tweaked' versions of 'dd' around which have a progress bar. I'm not a Mac user, so I don't know if these are available for your system. Alternatively, use NOOBS.