To access your Apps Script, go to Extensions -> Apps Script.
In order to run your script, you will need to save the project by clicking on the Save button.
Then you should see the Run button enabled. When running scripts for the first time, you will be prompt to authorized script to run on your google spreadsheet. Click on Review permissions to proceed.
You will be prompt to sign in with a google account if you are not already signed in. Select an account to sign in.
Once signed in, you should see this prompt to verify the app. Since you are the one that created the app, it is safe to go ahead and allow permission. Click on Advanced to proceed.
Continue by clicking Go to [project name]
Finally, click on Allow to allow permission.
