Mandarins usually eat copepods which are closer in size to baby brine shrimp. Normally, you need at least 100 pounds of live rock and/or a really productive fuge to support a mandarin.
Many LFS sell copepods in bottles. If you contact GreshamH, he can probably tell you where to go to find some.
If you do a reefcentral search, you'll find that some people have been able to keep a mandarin alive using other foods. From my reading and experience, that is the lucky exception rather than the rule.
Best of luck,
Roy