buying linden help

Hello, ok, I guess I'm being really stupid here, but I really need some help, Last night I thought I would purchase my first lot of lindens, but my L$ balance hasn't changed, when I have a look in my sl account it gives me a linden dollar balance and a US dollar balance (this is the money I transfered last night) how do I change my balence into lindens?

Sorry if I am being extremly dumb I have looked everywhere for the answer but I can't find anything to help me,

Thank you in advance for any help you can give x

on the "my account" webpage its on the left in the blue block, "Linden Dollar Exchange"

or you can do it in world by clicking the $ sign next to your balance