The AT89S52 microcontroller can sink current much better than to source.
Given this follow the simple connections as shown at

and connect the cathode(negative) of Led to port pin and the anode(positive) through a resistor to Vcc (+5V)
then when you make Low the port pin the Led should glow bright and when the port pin is High
then the Led should be off.
By this connection you make the port pin as a switch between ground and the Led.
Also have as a reference the datasheet of AT89S52 from Atmel at
http://www.atmel.com/dyn/resources/prod_documents/doc1919.pdf
at point 27. (page 29) Notes:

Notes: 1. Under steady state (non-transient) conditions,
IOL must be externally limited as follows:
Maximum IOL per port pin: 10 mA
Maximum IOL per 8-bit port:
Port 0: 26 mA Ports 1, 2, 3: 15 mA
Maximum total IOL for all output pins: 71 mA
If IOL exceeds the test condition, VOL may exceed the related specification.
Pins are not guaranteed to sink current greater
than the listed test conditions.