Tell a project to use the GPU

To tell a project to use the GPU, you need to add it through the project preferences. This is a per project decision, it cannot be a global decision. Not all projects use all GPUs, not all projects use any GPU even. Some only require the use of a CPU, others do only work via VirtualBox.

To use a GPU at a project of choice, log in on the project's web site and find Your Account. This can be disguised as your name in the top right corner, or a drop down menu called Account, or called Your Account on the front page. It can also be that the project links to it directly from the project command buttons in BOINC Manager.

When you have arrived in your account, look for the Preferences section. Usually the third option in is the project preferences. Click that.
Next click Edit preferences
Then click the check mark on the GPU or GPUs you want to use.
Then scroll to the bottom the the page and click Update preferences.

That's the project told that you want to use that GPU. Next you have to tell BOINC that, so it can ask for work for the new GPU:
Simple view: Open BOINC Manager->Projects: from drop down menu select Seti@Home->Project commands button: click Update.
Advanced view: Open BOINC Manager->Projects tab->select Seti@Home from the list->click Update.