if you create a pipe and put lids on both ends, does it work? if so, sounds like a model specific problem. if not, could be the way you're modeling or something about the graphics that are preventing you from making the selection.
could you elaborate on your trouble selecting the face? is it a selection problem? error message? also a picture of what your model looks like and what you can select and can't select might be helpful.
the only thing i can think of is if you have transparency enabled, have you tried removing that and then hiding components around it to make your selections or using select other?
By hiding the other components, I could easily select the side of the lid contacting the fluid.