You can find example assembly files and the microcode definition for
MAC-1 in ist/frend/one/ of the micmac directory.  Make sure you have
both a .mic file and a .mac file specified when you run sim.  Here is
a sample session that might help you:
 
sgi1% pwd
/home/chong/micmac/ist/frend/one
sgil% ../../meta/meta one.def
 processing instruction set definitions on one.def 
sgi1% ../../assem/assem one.def fact
 no previous file fact.mac.  Creating it
 successful assembly
 assembly object written on fact.mac 
sgi1% ../../ma/ma one
 no previous file one.mic.  Creating it
renumbering successful -- one.mal updated 
 successful compilation of microcode
 microcode object file written on one.mic 
sgi1% ../../sim/sim fact.mac one.mic
   ****** Welcome to the Instruction Set Testbed (IST) ******
one.mic read successfully. program length: 80
fact.mac read successfully. program length: 33
asm:quit