You'll need these:
vm_read_overwrite
vm_write
vm_region
You also need to use task_for_pid in mach_traps.h which gives you a "port" for the given PID, but you need to be root or add your account to the procmod group, as others have said.
There's an app called iHaxGamez which is good for scanning memory for ints and strings. Beyond that you're probably better off writing your own basic memory viewer, or use the one in Pocket Gnome (although it is rather limited). IDA can also be hooked up to OS X and works rather well, although you do need to run the GUI in a Windows virtual machine. The command line tool vmmap is also useful.
If you want a starting point for accessing the object list on OS X, have a look at
this thread.