change url of a tab using apple script
A way to do it with Chrome's built in scripting:

tell application "Google Chrome" to set URL of active tab of window 1 to "http://example.com"


And to expand your script to press return:

tell application "Google Chrome"
    tell application "System Events"
        tell application process "Google Chrome"
            set (text field 1 of toolbar 1 of window 1)'s focused to true
            set value of text field 1 of toolbar 1 of window 1 to "http://example.com/"
            keystroke return
        end tell
    end tell
end tell