When you download a stage for mmd how do you put it on the program?

Well, after you've downloaded and EXTRACTED it, you open MMD and at the main screen go to the little red box at the bottom that says Model Manipulation, click the drop-down button with the list of options that will say: Camera/Light/Accessories and the names of any models you have selected~ Go to Camera/Light/Accessories and click it~ You will see a wide yellow box to the right which says Accessory Manipulation~ Right below the drop-down box, you will see Load~ Click load and select the desired accessory, or in your case, stage~ Then a few other options will appear in the boxes below Load~ These are used to position, enlarge, enhance, or shrink the desired accessory~ And, if a stage is too small, go the the box that says "Si" at the bottom and backspace the 1 in "1.000 and put maybe a 3 or 5 depending on how big it needs to be~ If you knew that already, sorry, but it's better to kill two birds with one stone as apposed to~