Let's find and enable it for you.
Right click on the Start button and click on Properties
Right away we can see that choice to be made: "Start Menu" or "Classic Start Menu". The good news is that the one you're currently using will already be selected, and I'm going to assume you don't want to change this right now.
Classic Start Menu
Click the Customize button:
Make sure that the Display Run option is checked.
For the not-Classic version, once again click the Customize button. Then click the Advanced tab, and in the Start Menu Items list, and scroll down until you see the item Run command:
Make sure that's checked.
OK your way back out, and the Run... command should be visible on your start menu.