How can I use an IO ports on Digilent Genesys Virtex 5 to drive 6 external leds?
A Virtex-5 can drive up to 24 mA per I/O pin depending on the selected I/O standard and I/O bank voltage (see DS202, page 7 in Virtex-5 FPGA Data Sheet - DC and Switching Characteristics). The drive strength can be set in an ucf file.

Example ucf line:

NET "ML505_GPIO_LED[0]" ....... IOSTANDARD = LVCMOS33 DRIVE = 12 SLEW = SLOW;


See Xilinx Constraints Guide for more details.

If your LED needs a higher current, then use a FET as suggested by @diverger. You can use the ML505 Schematics as a reference how Xilinx connected LEDs to the FPGA (see page 10) . This board is equipped with dozens of NDS331N FETs (N-Channel Logic Level Enhancement Mode FET) to drive external devices. These FETs provide the currents to drive the LED and also act as voltage level-shifter. So a 1.8V I/O pin can switch a 2.5V LED. Using the NDS331N as a level-shifter for mid-speed data wires - as done on the ML505 board - is not a good solution! (e.g. I²C wires)