Ok, I put my memory stick in the USB port and it comes up in the top right hand corner a grey box saying something like this," You must remove at least one item from a USB port because the maximum amount of power has been used."
This isn't exactly what it says. I can't access anything which is on my memory stick on the PS3.
It's a PS3 slim 250gb.

i've checked the sony website and it says
The PLAYSTATION®3 250GB model does NOT feature: Backwards compatibility with PS2 games, Memory Stick card readers, SACD playback, Ethernet cable

so , unplug something from the usb hub
whatever is plugged in is powered by the usb port just like plugging into a wall socket and you're using too much power with stuff already plugged in .
if you don't have anything else connected then the usb device needs a powered hub , available at computer retailers and electronics stores for around $30