Set the network connection of your virtual machine to Shared Network or Bridged Network. You may need to select Disconnect before making the change to make the change visible to Ubuntu. Wait for Ubuntu to detect the disconnect, then change the network connection and Ubuntu will detect the new connection and IP address.
In a terminal window in Ubuntu, type "ifconfig" and make note of the inet addr which is the IP address of your virtual machine.
If you selected Bridged Network, then the IP address of the virtual machine will be on the same subnet as your Mac. (eg. if your Mac is 192.168.0.196 then the virtual machine might be 192.168.0.191 depending on the router your Mac is connected to).
If you selected Shared Network, then the IP address of the virtual machine will be on a different subnet described by the Parallels settings at Parallels Desktop -> Preferences -> Advanced -> Network: Change Settings... -> Shared. (e.g. an IP address between 10.211.55.1 and 10.211.55.254). All virtual machines using Shared Network will be in this range. If you type "ifconfig" on your Mac, you'll see it uses the vnic0 network adapter.
The IP address of your virtual machine should be accessible by your Mac. Test it with ping command line, or in Safari or whatever.
I'm not sure what a localhost web development environment is but it has to be accessible externally (outside the virtual machine) for your Mac to see it.
Network Connection type "Host-Only Network" is similar to Shared Network except that the virtual machine will be invisible to all external computers except your Mac and other virtual machines running on your Mac. Host Only Network uses a different subnet and range of IP addresses (see Parallels Desktop -> Preferences -> Advanced -> Network: Change Settings... -> Host Only). If you type "ifconfig" on your Mac, you'll see it uses the vnic1 network adapter.
