option 1: any upnp client app (plugplayer etc.), playing only formats (codecs and containers) which your ipad supports: http://www.apple.com/ipad/specs/
(it's pretty limited)
option 2: qmobile app by qnap, playing only formats which your ipad supports.
option 3: airplayer app by olimsoft (jonathan young), there exist also some other similar apps but i can't remember the names. it can play formats which your ipad doesn't support - however decoding happens on the ipad itself and performance is limited by its CPU (no hardware acceleration possible due to apple's restrictions) - so most HD content won't play, depending on bitrate, at least not without stuttering or slowdown.
option 4: using air video by inmethod. you need to buy their ios app and then you also need the free server software on your nas: qpgk for qnap x86 (intel cpu, NOT available for ARM i.e. your nas) nas only: viewtopic.php?f=234&t=44475
. it can also play formats which your ipad doesn't support - however the transcoding happens on your nas, and on-the-fly (live) transcoding needs A LOT of CPU power! so it's highly suggest you have a dual core nas (x59 Pro+/II), and even on those it will have problems with HD videos - only the 809 can maybe handle it (for sure the new x79, but those are surely "a little bit" too expensive for home use ^^). that's not a problem of qnap nas or the software, transcoding just needs raw power. you can also do "offline" transcoding with this software, if it's no problem for you to wait for some hours that will also work on a entry level x86 nas by qnap (for example TS-239 Pro II). instead of installing air video on your qnap nas, you can install it also on your pc or mac (which probably has a faster CPU, for on the fly transcoding) and let it have access to your qnap nas share, see http://www.inmethod.com/air-video/
- however then your pc/mac always needs to be on, as long as you want to have access to your files.
alternative solution: transcode/convert your vids on your computer first into ipad compatible formats and copy to your nas afterwards. for example a free windows software to do that: http://www.formatoz.com/