With the video in focus do you get a secondary option in the context menu 'Play using'? What does your playercorefactory.xml script look like and is the path to the player correct for your (what brand and how much ram?) box?
i don't get a secondary option and don't get play using
<playercorefactory>
<players>
<player name="MXPlayerFree" type="ExternalPlayer" audio="false" video="true">
<filename>com.mxtech.videoplayer.ad</filename>
<hidexbmc>true</hidexbmc>
<playcountminimumtime>120</playcountminimumtime>
</player>
</players>
<rules action="prepend">
<rule protocols="httphost" player="VideoPlayer"></rule>
<rule protocols="rtmp" player="VideoPlayer"></rule>
<rule protocols="ace" player="VideoPlayer"></rule>
<rule protocols="acestream" player="VideoPlayer"></rule>
<rule internetstream="true" player="VideoPlayer"></rule>
</rules>
</playercorefactory>
i am using kdlinks a400 and i have 2gb ram
the <filename> should include the exact path to the player; from the wiki it might be something like this<filename>Android/data/org.xbmc.kodi/files/.kodi/userdata/com.mxtech.videoplayer.ad</filename> (see note) or wherever you have the player
Userdata (wiki) and you should get 'play using' in the context menu. 2gigs might be cutting it close for this, especially with the free player that has advertisements.
Note: FTW Normally, you only need to use an external player if something is wrong with your Android device or you are doing something special/custom. Most users will not want to do this.
i am using it for the sound since kodi on android 5.1.1 can't use truehd