The steps are a little bit different than plugging in a camera or a USB key:
1) Connect the Eris (running) to the PC
2) Watch the Notification Bar at the top of the Eris' screen - you'll eventually see the little USB symbol there
3) Pull down the Notification Bar - you'll see a notification that says:
"USB connected (Select to copy files to/from your computer)"
4) Tap that (notification), and then press the "Mount" button.
At this point, use the PC/Mac the same way you would after plugging in a USB thumb drive - your files will be at E:, F:, G: or someplace like that.***
When finished:
5) On the PC (Windows presumed), use the "Safely Remove Hardware" Icon in the system tray to unmount the drive (as far as the PC is concerned).
6) On the Eris, pull down the Notification Bar, and select the notification which says:
"Turn off USB storage (Select to turn off USB storage)"
7) Tap the "Turn Off" button.
8) Disconnect the USB cable.
The reason that this seems way more complicated than using a USB thumb drive is that the Eris won't let it's computer use the SD card at the same time the PC has control of it. Steps 3 and 4 cause the Eris to relinquish control of it, and Steps 6 and 7 let the Eris have access to the SD card again.
*** I can't remember whether Windows auto-configures the device the first time through or not - you might have to walk through the usual USB driver install for Windows