Here are some quick things to know/try that work for me -
1. SD card must be formatted as FAT format, FAT32 is fine
2. Mod file is named update.zip in sd card, do not put it in any folder in the SD card
3. When you are powered down, if you hold the power button in for about 15 seconds and then release and then hold in again for a few seconds you will see, hold in for another 5 seconds to go to boot menu, immediately hold in for another 5 seconds and then release, you should see a android character, tap on home icon on right and it should go into a boot menu
4. Use the buttons on the tablet (home, etc on the right) to page down and up the list of selections. Top 2 buttons on the right are up and down. Second to bottom icon is select.
5. Scroll to "apply sdcard:update.zip"
6. Select that option (second to bottom icon on right of screen)
it should now apply the update.zip on your sd card. It could take a while.
If there are any errors then the update.zip file may be corrupted. All updates are over 90MB in size except for the patch that Dexter sent recently which is around 3MB.
Let us know if this works.