Theme Header and Mailchimp Popup

I am looking to add a popup mailchimp sign-up form to my website; however, when I follow the instructions form mailchimp to insert the code before the </head> tag Mailchimp is not able to detect the code. I was wondering if there was some restriction with the header capabilites since it is not the pro version or if I am just doing something wrong. Hopefully someone can help me.

Thanks