1 - First, you must download the 10.1 libraries for Aldec 8.1, not the
newest verion of the Xilinx libraries since they probably are for
version ISE 13.1. Make sure you do not download libraries for Aldec
8.3 either.
2 - If you go in the library manager in Active-HDL, check the
different Xilinx libraries and you should be able to see what version
of the fifo generator you have. For example, check unisim or
Xilinxcorelib, names similar to that.
3 - There is a global library file somewhere for Aldec, don't remember
where exactly, but if your library is missing in the library manager,
you need to add this library to the global library file.