Simply go to Pexels site visiting https://www.pexels.com then login your existing Pexels account or create a free account.
If you don't have an account click I want to download.
Or, if you are an existing user click Sign In.
If you want to create your account with your Facebook account click Join with Facebook. Or, enter your First name, enter your Email ID, enter a Password and click Create New Account.
Check your inbox. You will receive a confirmation email from Pexels. Click Confirm Email to confirm your email ID. In case you didn't find this email in your inbox don't forget to check your Spam or Junk Email folder.
You will be redirected back to Pexels. Click the drop down arrow next to your photo on top right and click API.
Click Get Started.
Under Request API Access enter a small description (for example; Using images in Explaindio Elements) enter your site URL (for example, if you have a website, enter that site URL or enter any of your public profile links), click the checkbox to agree to terms and click Request API Key.
On next screen you will get Your API Key. Copy the API Key.
Now open up your Explaindio Elements software and go to Settings.
Paste your copied API key next to Pexels API Key and click Close to close Settings.