Computer Mount

I recently got this mount for a present, the boxing was awfully damaged and looked to been open too. I started to put it together and that's when I noticed I was missing the piece for it to clamp to the desk(so you don't have to drill holes). I was thinking about doing it at some point but it would be nice to have all the pieces. Lastly after I got it all together I noticed that one of the screws that tightens down the arms to the post was completely missing. I had it all together, then went to put the mount to the arm and noticed that the arm immediately started to drop because of the weight, I hadn't even put a monitor on it. I tighten up the middle piece up until I thought it would tight enough, no the second the mount went on down went the whole arm until it my desk. So I ended up taking off the keyboard and only have one monitor on it with the middle as tight as it goes; it can still barely stay up on it own.