this easiest way to make sure your sd card is properly associated to your emulator instance is to create an AVD with an SD card. Do it as follows :
in Eclipse:
Window=> android SDK and AVD manager=>New :
-give a name to your AVD (no space allowed)
-give a SDK target
-give size to the wanted SDcard
-create the new AVD
Or, 2nd solution, you already have your own AVD and you want to add a SD card to it:
open a command prompt or terminal (windows/ linux?)
cd to your sdk/tools directory
execute the following :
mksdcard 256M NameOfYourCard
(you can pass the size you want ofc)
and then, put the following in you run configuration in the "Launch Additional command line" field :
-sdcard fullPathToYourCard
It should work.
If it's not clear, have a look in here for more detailled procedure