Hi Diane, I know where it is but when it comes to explaining directions to people I'm rubbish so I have found this for you
click on to get directions and put in that you want to go from Calle Lepanto it will then show you street by street. Make sure you click on the walking directions and not the driving one.
http://maps.google.co.uk/maps?hl=en&ie=UTF-8&q=Mercadona++in+benidorm&fb=1&split=1&gl=uk&ei=5ZHxScO7MsGg-Ab9nMHFDw&cid=10028752838433314481&li=lmd&z=14&t=m
It would take you about 20-25 mins to walk but you could get a taxi back. (The map actually says 17 minutes)
If you get lost ask for the Princessa Hotel and it's kind of just around the back of there or La Era Park is nearby too.
There is a Consum supermarket just 5 minutes away from the Trebol apartments though and they have a good selection of stuff at reasonable prices, you might be better shopping there, it's on Calle Ibiza next to Buddies bar..
Have a good time.
Sue