An easy way to set a macro with your local IP address, just add it as a build phase in Xcode.

set-local-ip.sh

#!/bin/sh
set -e

HEADER="${SRCROOT}/LearningSpaces-iPad/ip_address.h"

echo "// This file is generated by a build script." > $HEADER
echo "// Updated on "`date` >> $HEADER
echo "" >> $HEADER

ip_address="`ifconfig en0 | grep "inet " | cut -d: -f2 | awk '{print $2}'`"
echo "#define LOCAL_IP_ADDRESS @\"http://${ip_address}:3000\"" >> $HEADER