Need Help triggering 600ex-rt optically from mt24-ex

I have an mt24-ex that I like to use for macro work with my 100mm f/2.8 lens but I can't get it to trigger.

I normally use wireless on the my 600ex-rt(s) when I use multiple flash but can't do that if I want to use the mt24-ex.  It appears from manuals, web sites, etc. that I should be able to do this but so far I haven't had any success.  I did change channel (using channel 4) and ID setting (to 0000).

Must be overlooking something...any help is appreciated.
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Ben