Follow the following step to add your Adsense code to your website using pluging:
To do one: Log into your WordPress admin dashboard. Go to Plugins > Add new > Search for Insert Headers and Footers plugin. Install the plugin and activate it:
To do two: Go to Settings > Insert Headers and Footers menu. Paste the Google AdSense verification code into the Headers section. Save the changes:
To do three: Return to your Google AdSense account, check the box to confirm that you pasted the code and click Done:
To do four: Wait for Google to review your entire site and activate your account. For some websites the activation process can take a day, and for others it can take up to a couple of weeks:
Next Step is creating Adsense unit, read below article.