how do i optain a file from a usb memory device on my MAC

Put the USB stick in one of the USB slots on the back of the iMac.

Open a new Finder window (Finder>Command+N) - you should see your USB stick listed on the left under 'Devices'. Double click on it to open it, locate the file, click and drag it onto the desktop of your Mac or any location you want to store it.

To disconnect the USB stick click on the 'eject' symbol next to it in the Devices list and it should disappear from the Devices list. You can then unplug the stick.