First off, did you follow the instructions for the 64-bit install?
Second, are you running XLR(microphone) or DMX-512 cabling? Are you using a terminator? How long is your run?
Third: Are you booting with the MyDMX hardware plugged in? Don't do that. Boot, let services and apps load, plug in the hardware, wait for tones, run MyDMX. What do the lights on the hardware show?
Fourth: Are the Fresnels going to dimmers? Excuse my ignorance, but I'm mainly aware of these being on dimmers. That's not to say that they can't have DMX functionality built in, just that I haven't seen it, but I don't go looking either.
This sounds like a case of XLR cabling and a long cable run. I had a very similar issue 2 weeks ago at an event. I ran out of DMX cable and had to toss in a few short XLR's into a tree, then a long run of DMX-512 cabling. I should have just tossed a terminator into that last fixture and the backhauled the 200-foot DMX cable to the Opto Branch/4 and that probably would have taken care of my problems.