If the page is not loading in time, you can add waits - see http://watirwebdriver.com/waiting/. These wait methods will wait until either the condition is met or a time limit is exceeded. It is better than using sleep
since it only waits as long as needed (rather than waiting 2 seconds for something that loads in 1 second).
In this case, use the when_present
method on the span before clicking it:
require "watir"
browser = Watir::Browser.new
browser.goto "http://gis.nyc.gov/doitt/nycitymap/template?applicationName=ZOLA"
browser.span(:id, "zolaDisclaimerButton").when_present.click
puts "fin"