Use SpringBoard headers from rpetrich's repository from here https://github.com/rpetrich/iphoneheaders
Download these headers and put them in /var/theos/include. (If you're developing on your iDevice). Then type #import <SpringBoard/SpringBoard.h>
in your tweak.xm at the top and try to compile. Should work for you now.
Also look into rpetrich's theos fork. You should find the libsubstrate.dylib there