Root HELP:SU wont work :(


okay, so i followed this guide in all things root sticky titled "Updated: [08/31/2011] Inspire 4G, rooting - Windows/Linux/OSX"

to root my inspire 4g. and everything worked fine, so i thought. im able to use mobile hotspot no problem. so i was pretty happy about this, so i decided to find a rom and found that i need rom manager and superuser which should have already been on my sd card. i figured i would download them in the market, but that doesnt seem to work because rom manager wants to find super user in "system/bin/su" and in superuser, i tried to update it BUT it tells me to update it in recovery mode.
so i go into recovery mode and try to update and i get :

"" it says"Installing:""Finding update package...Opening update package...E:Can't open /sdcard/ aborted."

can someone help me solve this problem? ive tried everything i think... im also new to the whole rooting thing. (i also dont have sound right now) :/