If you are using the Excel plugin for the first time, you have to do a one-time installation.
To install Bloomberg Excel plugin, close all Excel spreadsheets and follow this path: Start > All Programs > Bloomberg > Install Excell Add-in
Once installed, the Bloomberg tab will appear in the Excel ribbon.
If there is an error in installation, run Start > All Programs > Bloomberg > Bloomberg API Diagnostics > Start & Repair, and try to install the add-in again.