I use this, it seems to work nicely.
<TransitionsAndEffects Version="1.0">
<Effects>
<EffectDLL guid="{ADEADEB8-E54B-11d1-9A72-0000F875EADE}">
<Effect name="Freeze Frame" iconid="31" comment="Freeze Frame">
<Param name="FrameSkips" value="10000" />
</Effect>
</EffectDLL>
</Effects>
</TransitionsAndEffects>
Copy the above code and paste it into notepad, save it as "Freeze Frame.XML" or similar. Be sure to add - .XML - to the name or else it might not work. After you have saved it, go to Program Files > Movie Maker > shared. Make a new folder within the shared folder and name it "AddOnTFX" Put the file in the new folder.
Restart WMM, and go to Tools > Video Effects, and search for "Freeze Frame" Click on the thumbnail and drag it onto the clip you want to freeze. It will freeze on the very first frame of the clip, and stay frozen for the duration of the clip.
hope it was helpful.