You have not correctly set up a call to LINGO's numeric integration function, @INTEGRAL. The correct usage is:

@INTEGRAL( PROCEDURE, X, LB_X, UB_X, Y);

where PROCEDURE refers to the name of the procedure containing the function to integrate, X is the variable to integreate over, LB_X and UB_X are the lower and upper bounds on X, and Y is the variable that stores the value of the function being integrated.  Refer to the @INTEGRAL documentation for more information.