To attach an EA, please follow these steps:

• Open Navigator Box (Ctrl N)
• Expert Advisor
• Right-click on EA that you want to use
• Attach to chart

And in case you are willing to apply for a purchased EA from the MQL site,

Please follow these steps:

• Go to the “File” tab in the menu bar
• Click on Open Data Folder
• Open the “Experts” folder in the “MQL” file.
• From amongst all the files, Choose the one you want to apply
• Reopen the trading platform.