See the wiki
http://plugcomputer.org/plugwiki/index.php/Factory_Default_u-Boot_Environment. I ran into the same problem. The wiki could probably use some better structure but all the info is there

In short, after doing a resetenv, you'll need to do at least:
setenv bootcmd 'nand read.e 0x800000 0x100000 0x400000; bootm 0x800000'
setenv bootargs 'console=ttyS0,115200 mtdparts=nand_mtd:0x400000@0x100000(uImage),0x1fb00000@0x500000(rootfs) rw root=/dev/mtdblock1 rw ip=10.4.50.4:10.4.50.5:10.4.50.5:255.255.255.0:DB88FXX81:eth0:none'
setenv ethaddr 00:50:43:4c:02:26
setenv run_diag no
saveenv
Be sure to set the ethernet address to the one on the sticker on the back of the plug. Don't just copy/paste above.