Since I downloaded Ubuntu One my side tab for Ubuntu Software Center is missing.
Please Open your terminal (CTRL+Alt+T) and type
Check your /usr/share/applications and ~.local/share/applications folders for the file "ubuntu-software-center.desktop". If it does not exist, do the following in terminal:
Type in your password, and paste the following into the text document:
Name=Ubuntu Software Center
Comment=Lets you choose from thousands of applications available for Ubuntu
Save the file to either /usr/share/applications (recommended) or ~.local/share/applications. Then press Alt+F2 and type unity --replace.
Let us know if the file is back