You need a file manager application to gain access to your SD card. They are available on the Android Market if you have access. Astro is very good.
If you are in a location that doesn't support the market you could try a couple of other things....
First have a look at this link, it says you can manage apps via your PC so you could presumably download the file manager on your PC then load it on to your phone. http://talk.sonyericsson.com/message/24054#24054
You could try getjar.com or SlideMe.org accessed via your phones browser and find a file manager that way.
Hope it helps.
