Well, XBMC has different types of addons, and I guess this one is a video addon that is not really "executed" per se.
So probably you want to use GUI.ActivateWindow and then it is just a matter of finding the right parameters.
Here's a similar command for a video addon (newznab looked way too complicated to install, sorry)
<?xml version="1.0" encoding="utf-16"?>
<command id="416" name="open nasa videos" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>XJson.Raw</cmdType>
<cmdString>GUI.ActivateWindow&&"window": "video", "parameters": [ "plugin://plugin.video.nasa" ]</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>open nasa videos</phrase>
</command>
Ideally when you want to post a command you could right click it in your tree and select "copy" and then paste it into a code block on the forum, as I have just done. then others can simply copy the code block and paste it into a group in their tree. (You can copy commands, or entire groups this way).
I can't test it but hopefully this command will work for you:
<?xml version="1.0" encoding="utf-16"?>
<command id="422" name="open news nab" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>XJson.Raw</cmdType>
<cmdString>GUI.ActivateWindow&&"window": "video", "parameters": [ "plugin://plugin.video.newznab" ]</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>open news nab</phrase>
</command>