For clarity - even though these PCIe slots are longer than the ones i'm used to seeing, they are still in fact PCIe slots and the card should work. Right? They should take this 64 bit style PCIe card.. I have seen several online tutorials of people installing cards with the same pinout as this in 2012 Mac Pro towers.I'm not familiar with that particular card, but just googled it and it appears to be an audio interface. Says it's compatible with the Mac Pro, but I couldn't find anything more specific than that--like what model years of the Mac Pro. Have you tried contacting them to see if they have any guidance?
About all I can suggest is to try it in every slot (except for Slot 1 which I assume has your video card) to see if maybe you get a different result. You could also try a PRAM reset and see if that helps any.
[doublepost=1539642368][/doublepost]SOLVEDYes if the card you have is PCIe then it should fit fine into any of the four slots in your Mac Pro. Whether it's compatible with the 2010 Mac Pro has other variables, so that's why I think contacting the manufacturer to ask would be a good step.