Here is the solution: in Pre-logic Script Code Box:
def foo(MamerMN):
if (MamerMN <= 0.200000):
return 1
elif (MamerMN >= 0.200001 and MamerMN <= 0.400000):
return 2
elif (MamerMN >= 0.400001 and MamerMN <= 0.600000):
return 3
elif (MamerMN >= 0.600001 and MamerMN <= 0.800000):
return 4
elif (MamerMN >= 0.800001):
return 5
and in Expression box
foo( !MamerMN! )