AdvancedHMI can read and write to tags that exist in the PLC. It cannot modify the program or configuration of the PLC. It's a HMI software package, not a program development package.
If you need to upload the program or make modifications to it, you have only one option. RSLogix.
If you happen to have a copy of the .ACD file somewhere, I could take a look at it for you.
The most likely reason you are getting the Privilege Violation error is because when someone created the tag, they configured it to read only (b-hole move), otherwise AHMI would be able to write to it. The problem is not AHMI, no software package would be able to write to that tag when it is configured as Read Only.
As for the price, it's pretty expensive. You have to go through a distributor to get it, though if your distributor is nice, they may be able to offer some assistance. They have a "mini" version (or at least they used to) which allows you to upload/download and view logic online, just doesn't allow you to make changes online. It also may only support ladder logic, I can't remember.