Do you have one of the blue, 4-axis or 3-axis boards?
In the first post of the thread at http://www.cnczone.com/forums/genera...y_chinese.html, there is an attached file named schematic_controller_TB6560.zip that contains schematic diagrams for a 4-axis board (you should double check to make sure that your board is wired in the same way as the OP's board). If you unzip that file and look at Slide01.JPG, Slide02.JPG, and Slide03.JPG, you can trace out the wiring for the spindle relay control.
It looks like pin 2 of the DB-25 connector is used for spindle control. I'm not sure from the schematic whether that pin is "active high" or "active low", but it should be easy enough to test. A "high" (+5 volt) signal on that pin will probably energize the relay "RLY61", but if not, then a "low" (0 volt) signal on that pin should do the trick. The spindle relay input line should also be present at pin 10 of the DB-15 accessory connector (but if you want to control the spindle from Mach3 or other software, then you would probably use the DB-25 input instead of the DB-15 input).
It looks like 2 relay contact pins are taken to an output connector on the board (the 4-pin green connector on the edge of the board, close to the black relay). The other two pins of that connector are supposedly connected to ground. I do not know what the relay output contact current or voltage rating is (but maybe that is printed on the relay itself).
As far as I know, the blue Chinese TB6560 driver boards do not have the capability of putting out a variable control voltage to set the VFD spindle speed. Some other drivers (such as the Geckodrive G540_ have a 0 - 10 volt analog output for that purpose.