You also have a space after the ngrp: prefix. Sometimes the forum adds spaces, but I think that is what you pasted in:
http://s4.tilda.com.tr/live/ngrp: onur_all/jwplayer.rss
It should be:
http://s4.tilda.com.tr/live/ngrp:onur_all/jwplayer.rss
which works:
<rss xmlns:media="http://search.yahoo.com/mrss/" xmlns:jwplayer="http://developer.longtailvideo.com/" version="2.0">
<channel>
<item>
<title>onur_all</title>
<description>onur_all</description>
<jwplayer:streamer>rtmp://s4.tilda.com.tr/live/_definst_</jwplayer:streamer>
<media:group>
<media:content url="mp4:onur_360p" bitrate="663" width="640" height="360"/>
<media:content url="mp4:onur_270p" bitrate="249" width="480" height="270"/>
<media:content url="mp4:onur_audioonly" bitrate="49"/>
</media:group>
</item>
</channel>
</rss>
Richard