To be able to operate all switches and buttons in DCS, the data of your aircraft must first be imported.
Choose Edit --> Import DCS Data from the menu.
You will then see the following pop-up screen appear.
Click on "File location" to indicate the location where DCS or DCSbeta is installed.
Then choose which plane you want to import from the drop-down menu.
Then click on "import" to import the selected data.