For the falls, I didn't use the fake rock or a piece of flag stone. I cut the lip off that it was suppose to set on.
I did leave the fake rock for the skimmer. Just because it is easier to clean the skimmer.

 

so you just cut the whole lip off adn let the water just kinda spill out? HOw did you get the water to fall in to the pond to make the needed sound effect or did you do this for a stream type appliation?

 

We always cut the plastic lip off of the biofalls and use a piece of flagstone or angular stone to create the waterfall. If you do cut the lip off make sure to cut it off before you fasten it on or immediatlaly after you fasten it. If not, the pressure you apply while cutting it off could break the seal.
As for as the skimmer we usually keep the plastic lid on because it is light, making access easier. We have used several pieces of flagstone placed over the the top. We also will place grasses in this area to help hide it. Skimmers are hideous!

 

This is as close of a picture as I have. It makes plenty of sound.

Like Tom said, if you cut it off, do it right away so the seal dosn't brake.

 

Disguising the skimmer lid is easily accomplish using two methods. #1 (my personal favorite) place a wide clay pot with a draping annual or fern that fans out and low over the skimmer top. This will hide it nearly completely and the pond owner doesn't have to move a heavy flagstone to clean the skimmer.
#2 thin slate, remember keep the wait down, most owners don't like to lift heavy rock to clean the skimmer thats why they hired you!

I'm working on another method by which you spray the lid with resin or glue and sprinkle mixed gravel on it. Prehaps I'll use mulch?