The picture below shows the metal prop rod that typically comes with a game and its location which is usually found on the right side of the game box. The playfield can often get rather "torqued" if this rod is used, but hey, the factory put it there to hold the playfield up, so I guess it's OK. Personally, I use a wooden prop rod to hold up the playfield, or I carefully raise the playfield to its most upright position leaning against the Backbox. Remove the ball from the game since you won't want it to go crashing in to playfield plastics and breaking something.
(click picture to enlarge)