SharePoint 2010 is available in 2 versions namely Standard and Enterprise. If you have installed a standard version and want to convert to Enterprise features then you can simply change the license type by
- Opening Central Administration
- Entering the new License Key by Clicking Convert farm license type
Looks Simple.. But wait, Do you see the Textbox disabled? 
There is another way to change License type from Standard CAL to Enterprise CAL.
- Open Central Administration
- Navigate to Upgrade and Migration
- Click “Enable Enterprise Features“
- Enter your Enterprise CAL key and Press OK
- Wait for few minutes and enjoy the Enterprise features of SharePoint 2010
Keep in mind there is no turning back here. Once you upgrade to Enterprise version you cannot switch back to Standard version.
