The L293D pins 10 & 15 are the 3A & 4A inputs.
I'll assume you have the motor connected between pins 11 & 14 (3Y & 4Y) outputs.
Pins 4,5,12 and 13 to ground.
Pin 16 (Vcc1) to +5V (would work with +3.3V, however the logic inputs A's and EN's will work from a 3.3V uC)
pin 8 (Vcc2) to the motor power supply or battery.
pin 9 (EN) tied high.
This is from the function table and circuit diagram on page 2 of the data sheet.
The other H-bridge (1 & 2) are not connected (pins 1,2,3,6,7,8).
Ok, now back to figure 9 and the truth table on page 9.
EN must be high else motor = stop.
If both 3A & 4A = high then motor = fast stop.
To run the motor 3A != 4A or if one is high the other must be low.
Which is what your code does:
PORTB=0x01; // forward 0001, 3A = H, 4A =L
PORTB=0x02; // reverse 0010, 3A = L, 4A = H
Check the L293's pins 10 & 15 with a voltmeter. They should be toggling every thirty seconds.
If not check that you have PORTB pins setup as outputs.
Do you have the L293's grounds connected to your uC's ground?
Do you have the L293's EN pulled high?
Is the voltage on VCC1 = +5V
Is the voltage on VCC2 enough for your motor?
Recheck these and let us know what you found.