PayPal API - Enable this solution stays greyed out
I'm quite new to Magento and I'm having a problem enabling PayPal Express Checkout .
Under System > Config > Payment Methods I've added the relevant PayPal API credentials and set API authentication method to API Signature but when I save the config the 'Enable this solution' dropdown remains greyed out.
I'm using Magento 1.9.2 with onepage checkout
Anybody had the same problem - am I missing something?