It is hard to tell what is going on on your system, but I hope that the following will be helpful. First, let us start with a new session or server. So we have a blank document (standard interface, worksheet mode in the following example) and a fresh server associated to it.

Step 1: From the Insert menu, select Plot, and then 2D.

Here is what you should see after this step:

Step 2: Enter the expression sin(x) in an input region.

Step 3: Select the full expression in the input region and drag it into the plot region.

On the above screenshot, note the form of the mouse cursor when it is above the plot region, especially the small empty rectangle on the bottom right.

Here what should appear on your screen after releasing the left button. Note that the original expression in the input prompt has disappeared.

Now, say we want to add another expression and also keep it in the input line. To do so, we enter the expression as usual but we also use the Ctrl (control) key when dragging the expression over the plot. Note the plus sign that appears inside the small rectangle at the bottom right of the mouse cursor (if you release the Ctrl key, the plus sign disappears. If you hit the Ctrl key again it comes bask).

And here is the final result.

HTH,
--
Jean-Marc

As mentioned before, this is a bug in Maple 11.01. It should be fixed in the next dot release which will be available shortly.

To check which version of Maple you are running go to Help -> About Maple.