create a text file in your home directory called ".gdbinit". Give it the following contents:
source /usr/libexec/gdb/plugins/MacsBug/gdbinit-MacsBug
Make sure that your terminal window is nice'n'large and type 'mb'. Type 'mb' again to get out. 'help macsbug' to see your newly installed commands (mostly stuff for stepping through ASM).
If you use the step commands ('so' and 'si', you'll get a live register display without needing to enter the 'mb' mode.