You can follow http://forum.xda-developers.com/show....php?t=1622425
and use Blackthunder's tool if its ok with you. Or I can tell you one more method:
1. Download After OTA from forum section. Search for it you will find a .zip file of about 103 mb.
2. Download and install the Acer a500 drivers.
3. Attach your tablet to the computer.
4. Unzip After OTA and run it.
5. From standard tab select your desired Bootloader and recovery, you can select any except HC Bootloader for running ICS. Remember to select A50X.
6. Remember if you are having ICS rom its OK, but if you do not have ICS Rom than keep one in your sd card to flash after installing the Bootloader.
7. You might also brick up your tab if anything went wrong so be carefull.
"Thanks to the original developers for the After OTA tool and Guides available."