Are you supplied with a static IP from your ISP? If you ware you would likely want to make sure your MX is set up in NAT mode and you would then use the static IP settings from your ISP to configure the WAN on your MX. (That would mean the static IP from the ISP is your public IP)
If your core Cisco switch has a default route pointing at the ASA's "inside" (LAN) IP, then you can replicate that on the MX using the "use a single LAN" option. You would create a "Local LAN" that matches the ASA's inside IP and then create static routes for all of your internal subnets and put the Cisco switch IP as the next hop.