How many drives do you have in Fusion?
Windows has a tendency to get confused, requiring manually assigning drive letters.
Try turning on USB 2.0 support again and try this.
Right click on My Computer, click Manage. Click Disk Management in the left hand pane.
Scroll down the list of drives in the right hand pane and see if you can see your flash drive there, it helps if your flash drive has a unique name.
If your drive is there and it has a drive letter, see if it has one that's already taken under My Computer for something else. If it isn't taken by anything else and it still won't show up, click in the address bar and type the drive letter it is assigned followed by a colon i.e. F: and press enter.
If the drive has the same letter as something else, in Disk Management, right click it and click 'Change drive letter or path'. A dialog will appear, click change. Select a new letter, I usually choose M: since it's rare for that to be used by something else. Click yes at the warning about changing drive letters potentially causing problems.
Hopefully Autorun will run or it will show up in My Computer.