I just purchased a pair of these myself and was a little perplexed at the same thing - this being my first pair of bib shorts (of any kind). I found it works if you pull it down as far as you can, then bend over as far as you can. Does that make sense? I kept hold of the fabric as I was concerned about wetting the shorts - but it worked fine.

Have only done this once and it was quite a challenge - but then again I find it difficult to walk and chew gum at the same time...