I took a look at the Brando shop (
http://shop.brando.com.hk/aximx5.php) and didn't find any batteries on sale for the X50/50v. Are you sure you got it from there?
EDIT
Oops, I take that back. Found the site (
http://shop.brando.com.hk/pdabattery.php). I'm not sure why it won't start. Have you tried charging the battery separately on the cradle? Also, when you plug in the battery in your Axim to charge in the cradle, can you turn the Axim on?
Another thing - I'm not sure if it relates to the cause of your problem but the basic Axim batt is 1100mAh. I doubt the 1000mAh batt would make any diff other than to give you slightly less batt life but still, it's something to consider. I assume your Axim works fine with the standard 1100mAh batt, right?