What should I do if the plastic waterline connection on my dishwasher is stripped?

I had the same problem! Saw your pic and it exactly like mine-plastic piece on dishwasher and you have to add a metal elbow. They don't marry well. After several futile tries, and reading what happened here, I think the plastic wasn't stripped-it just doesn't match well. So I took the elbow off the hose (much easier to manage) and wrapped a bunch of plumbers tape around the blue plastic female part. I then started attaching the metal elbow by hand, and moving with a wrench. I kept on tightening it until it was almost to the top,and wouldn't move any further. Then I attached the hose to the other end of the elbow. No leaks!!! I think the plumbers tape did the trick. I had put that thing on and off several times before looking for other solutions.

Yeah, this seems to be a common occurrence. I had exactly the same problem. I had to put the thing on the belt sander to remove about an 1/8th of an inch of plastic. I had to completely remove the plastic piece and then I was able to put it on correctly. I have also ordered the spare part which is 17 bucks on Amazon. I'll swap it out later... Good luck if you have this issue. Whirlpool needs to rethink this part or include the adapter that is required.

If I was doing it over again I would have assembled the elbow to the inlet valve before installing dishwasher.everything exposed and easier to make connection without crossthreading. Now that I already crossthreaded the conn this worked for me.I disconnected everything and moved the dishwasher out so I had easy access to valve. Removed valve. Took a hacksaw and cut off 2 or 3 threads. Installed elbow.worked perfectly.

In case it helps anyone, here is what worked for me. I had tried two or three times to tighten the elbow piece into the blue plastic. But even as I was getting it tight with a wrench, it looked like it was not turned in enough. So the next time I made a conscious effort to push the piece up when I started threading; it did go up more turns before it got tight, and tightened it a bit more with the wrench, and it's not leaking now.