Can you describe "Not Working" ?
Is there some error?
Does nothing at all happen?
Does it do something different from what you expected it to do?
I have noticed sometimes that the cursor in the reply window doesn't move to where I put it.
But If i go ahead and type anyway, the new text does go where I intended it to.
Maybe it's something similar to that.
What happens if you type anyway after pressing enter, does the new text go to the next line?
It may be a graphics issue, try checking/unchecking the "Use software rendering instead of GPU rendering" in Advanced IE options.
I generally use Chrome but just tried IE10 and noted the same issue.
Apart from clicking the Compatibility button I think there is another option.
For me the problem only occurred when I have my Message Editor Interface set to "Enhanced Interface - Full WYSIWYG Editing" (N.B. This is not my normal setting)
Is that your setting?
If so, (without Compatibility mode on) in the Quick Reply window, click the top left icon to "Switch Editor to Source Mode" and see if that activates the Enter key action. It does for me.
If I re-click that icon to "Switch Editor to WYSIWYG Mode" the Enter key stops working again.