The simplest script for failover I know, (for outgoing connections only) is just setting a different route distance for the default route for both ISP. If you get an IP address by DHCP from the ISP or from the modem, you can set the default route distance in the DHCP client of the interface.
The advanced (and much better way) of doing it is here: http://forum.mikrotik.com/t/advanced-routing-failover-without-scripting/136599/1 (It even does load balancing)