You can follow the steps below to enable it and speed up the conversion. 
1. For NVIDIA® CUDA™, AMD® technologies
1) Follow the steps below to check the processor of the graphic card on your computer is included in the following link.
Click Start>Run and enter "dxdiag".
Click OK to open the DirectX DiagnosticTool window. You can check the type of the graphics card and its driver under Display tab.

2) Make sure your graphics card has a latest driver, if not, you can download the latest driver for your graphics card from the link below for installation.
For NVIDIA® CUDA™ technology:http://www.nvidia.com/Download/index.aspx?lang=en-us
For AMD® technology: http://support.amd.com/us/gpudownload/windows/Pages/eyespeed_downloads.aspx#1

3) While output to video formats including h.264 encoder, enable the GPU accelerate feature by clicking the Video Editor icon on the top left corner of the program and check Enable GPU Accelerate option under the pop up Settings window. 

2. For Intel quick sync video Technology
If you are using Intel graphics card, the product will perform GPU encoding with Intel Quick Sync Video technology, which is built in 2nd generation Intel Core Processors or above only. You can download the latest driver from the link for installation for a better GPU encoding performance. If you are using 1st generation Intel Core Processors, you can download Media SDK 2.0 from the link.

The operation steps to enable GPU accelerate are similar to that of NVIDIA® CUDA™, AMD® technologies above.