[OS X TeX] MF display variable

[Steve Peter]

I'm back in my periodic MetaFont interest phase, but on a different  
machine from the last time. Now, on my MacBook Pro, when I attempt to  
run mf, I get

hrothgar:~/Documents/Art/FontDevelopment/MetaFont speter$ mf io
This is METAFONT, Version 2.71828 (Web2C 7.5.6)
(io.mf
The letter O [79]Error: Can't open display:

What do I need to set and where?

Thanks!

Steve

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