it could be a very small error, so it may be necessary to doublecheck the basic settings. For reference, here’s my (working) configuration on a Win7 (64 bit) system:
when connecting the controller board via USB cable, the “USB Serial Port (COMx)” shows up in the device manager. Driver version is 188.8.131.52 from FTDI, 27.01.2014. Connection settings there are 9600 bits per s, 8 data, no parity, 1 stop (aka “8 N 1”), and no flow control. Device status is “… working without problems” (or likewise, translated).
In Repetier Host, Printer Settings, Connection tab, options are: Serial connection, COM port matching the device manager, baud rate: 250000, transfer protocol: autodetect, Reset on C.: disabled, Emergency reset: send emergency stop + reconnect, Cache size: 63, ping-pong communication: off.
Note especially the cache setting; iirc I’ve seen a version of RH, where the default setting was still 127 bytes.
Clicking the Connect button (still no power on the controller board), it should turn green, and the blue box in Manual Control tab should change from “Connecting” to “3 Commands waiting” or similar. The number should increase if you click any of the buttons below, e.g. plus 3 for any “arrow button” axis movement (XYZE), or plus 1 for all other commands. Controller board should still show two USB LEDs (red/green) constantly lit.
When switching on the 15 V power supply, the signal that you already saw should appear, i.e. USB LEDs going dark, LD1 blinking twice, then USB LEDs flashing fast during data transfer.
Below the last log line (probably “Using fast VBOs for rendering …”) in RH, several new lines should be written, starting like
13:11:19.756 : start
13:11:19.756 : echo:PowerUp
13:11:19.756 : Marlin 1.0.0
Several errors may be present due to the commands waiting, mainly “Error:Line Number is not Last Line Number+1”; you can ignore those for the time being. Above the log, enable “Commands” and “ACK” to see all communication. With the regular temperature checking enabled in the Printer Settings (Printer tab), you should see M105 commands and acknowledgement-messages with the extruder and bed temperatures.
Probably, at some point your configuration will differ. Where?