External Clock for DE2 board

There's no need to do this. Just add a PLL to your design from the IP Catalog (in older versions of the software, it was the MegaWizard Plug-In Manager) and set it to divide by two. Connect the 50 MHz input clock pin of the board to the input reference clock of the PLL and the output to whatever logic you want to clock at 25 MHz as well as an output pin of the device. Done.