How to access /dev/mem in OSX?
According to Accessing Kernel Memory on the x86 Version of Mac OS X, /dev/mem and /dev/kmem were removed when Apple released OS X for Intel processors.

The (excellent) article also explains how to reenable the memory devices, namely using the kmem=1 kernel boot argument. 

Note that, as commented below by neal, in macOS 10.15 Catalina (and probably since macOS 10.12 Sierra), NVRAM settings can only be changed when your Mac is booted into recovery mode. To enter Recovery mode, power on your Mac and immediately hold ⌘-R until the logo appears, launch Terminal from the Utilities menu in the menu bar, run:

sudo nvram boot-args="kmem=1"


and reboot. You should see these two devices:

$ ls -l /dev/*mem
crw-r----- 1 root kmem 3, 1 2014-02-28 22:09 /dev/kmem
crw-r----- 1 root kmem 3, 0 2014-02-28 22:09 /dev/mem


(In previous versions of macOS like OS X Mavericks 10.9.2, you can reenable the memory devices within the operating system, so simply launch Terminal, type the command above and reboot.)

If your Mac won't boot or you have any issues, reset the NVRAM by pressing ⌘optionPR and holding the keys down until you hear the startup sound for the second time, as explained in "Resetting NVRAM / PRAM" in KB HT1379 About NVRAM and PRAM.

If you wish to reset the argument, type sudo nvram -d boot-args and check that nvram -p | grep boot-args does not print any results.