I thought I will create a HOW-TO with step-by-step instruction (+ some screenshots) how to sort this problem out.
Have a look at here -> http://stonebreaker.calka.org/index.php?id=how-to-fix-a-problem-with-trash-dustb in-icon-in-mac-os-x-shown-always-empty
For those who don't trust the external sources I placed the article below. There are 2 options worth to try, which should resolve the issue:
Option I
Before you start, make sure your trash (dustbin) is NOT empty. If it is, just create new folder on your desktop and move it to the trash (dustbin).
Start the Terminal application (Applications -> Utilities -> Terminal.app) and run the below command:
defaults read com.apple.dock trash-full
It should return '1’ (but you get '0' I guess). If that's the case run the command below:
defaults write com.apple.dock trash-full -bool TRUE
and next the below one to check again:
defaults read com.apple.dock trash-full
you should now get '1' correctly.
Empty the trash (dustbin) now and put something into it. The icon should change to the full. Empty trash (dustbin) and check the icon again.
I hope everything is ok now. If not, this might be an issue with the permissions of the Trash (dustbin). To fix this just finish the steps from Option II.
Option II
To fix problem with permissions use Disk Utility to restore permissions to the folders across the computer.
Start Disk Utility application (Applications -> Utilities -> Disk Utility.app)
Select the disk your Mac OS is installed onto (usally named Macintosh HD) and under First Aid tab press Repair Disk Permissions
That's it - I hope this has resolved the issues.