I'll pretend you haven't done anything...
Go the the specified site and D/L the program. Then, from the original post, D/L the Common.rar.
Install the email program and pay attention to where on your computer it's installed. Unzip the Common.rar file on your desktop. Once unzipped, it should have a file called common.dll inside.
Navigate to the location where the program is installed, likely C:\Program Files\whatevertheprogramisnamed. Locate the common.dll file that came with the program. Make the window smaller, but don't minimize it to the task bar.
Now, open the folder that has the cracked common.dll file, also in a small window, so you can see them both.
Drag and drop the cracked common.dll file into the legit program folder. It'll ask if you want to replace the existing common.dll file. Click "Yes."
Open the program and enter any 10 random digits to register it.
HTH