I have a similar problem to the original poster.
In dmesg I get:
sdhci: Secure Digital Host Controller Interface driver
sdhci: Copyright(c) Pierre Ossman
mvsdmmc: irq =28 start f1090000
mvsdmmc: no IRQ detect
and there is never any device with a name mmblk* created in /dev when I insert an SD card.
I am using the 18.104.22.168 kernel that shipped with the plug (I know this doesn't work for booting from the SD card, but I am just trying to follow the wiki instructions to create a root filesystem).
Is there something more I need to do, or is the plug broken, or what?