Tables are not too convenient in LaTeX. In fact, it is hard to get a specific combination of features to work. This hint is on using a rotated tabularx table within a memoir document that should feature footnotes under the table.
The basic idea which is frequently applied and commonly documented, is to put the tabularx environment within a minipage (which even works within the sideways environment of the rotating package). This works pretty much for all packages (including memoir) as long as hyperref is not loaded. This is because hyperref typically has problems with the footnotes of tabularx (which are placed in a single line below the table) and, hence, disables the footnote output in tables.
In this scenario, hyperref is too cautious, as putting tabularx in a minipage is all you need in order to get working footnotes. Therefore, you will have hide tabularx from hyperref. Technically, memoir does not load tabularx but rather emulates it, but this is neither an issue for hyperref nor for this solution. Just add the following line to your preamble before loading hyperref:
\expandafter\let\csname ver@tabularx.sty\endcsname\relax