Basically, Shareport isn't going to do you any good.

Let me get real specific for you, because I believe there is a misunderstanding of the (somewhat confusing) topic of what UPnP is.
The DIR-655 does
not contain a UPnP-AV server. It is very important to understand the type of UPnP the 655 supports works in terms of allowing UPnP-aware network applications (utorrent) and appliances (DSM-G600) change specific router settings to allow proper communication to the internet.
The DSM-G600 contains a UPnP-AV server. This allows the G600 to say to UPnP-AV aware applications (Windows Media Player 12) & appliances (DSM-750) 'hey, I'm sharing these folders & these files' and vastly simplifies the media sharing experience.
Shareport was designed to make any USB device that is plugged into the DIR-655's USB port wirelessly appear on a Windows computer as if it were physically plugged in. Since the DSM-700 isn't a Windows computer, you can't install Shareport on it, and you can't benefit from it.
ECF is right in saying you can patch the G600 to allow MKV to work, but it's work and not really something I'd advise anyone short of a Linux super-user to attempt. If "make" "configure" and "nano" mean nothing to you, don't bother with it.
I can certainly understand why UPnP can be a confusing topic as well as DLNA (which is a subset of UPnP features). Honestly, the easiest thing would be for D-Link to upgrade their NAS products to share MKV over UPnP - especially since their playback devices support this feature. But this isn't the forum for that.