First off, what error are you getting?   In looking at your if statement, be aware that Crystal does not work quite like BASIC and I have noticed odd results if you do not put parans around code of the then of the if statement (also on the else, if a multi-statement).

if {USER_DEFINED_DATA.ASSESSMENT_MONIKER} = "4D17FC8EFA484E8B884E5F1C9B3D55DB"

then
(
stringvar x;
numbervar start;
numbervar end;

 x:={USER_DEFINED_DATA.ASSESSMENT_DATA};
start:= instr(x,'@@T_400_ = ') + 12;
end := instr(x,'"@@T_501');
x:=mid(x,start,end-start);
x
)
else
{USER_DEFINED_DATA.CAD751}