Multibit classic used to have an option in its multibit.properties file to work with the testnet, but I don't know if it's still working.

Quote from: jim618 on November 30, 2012, 10:04:02 AM

Hi Binford 6100,

To use the testnet, there is an option that you put in the multibit.properties configuration file.

I see you are on a Mac, so I recommend doing the following:
1) Duplicate your multibit.app to, say, multibit-testnet.app
2) In Finder, do a right click and 'Show Package Contents'.
3) Navigate to: /Contents/Resources/Java
4) If you put a 'multibit.properties' file in the Java directory that install of multibit (only) will pick it up and use it. It effecively runs in local mode and does not use your user data area.
5) Create in TextPad a file called multibit.properties and save it to the Java directory.
6) In the file add the line:
testOrProductionNetwork=test
(case sensitive)
7) Then when you open that install of multibit you will connect to the testnet and see something like:
http://multibit.org/postImages/testnet.png

A couple of things to note:
a) I almost never use the testnet so you might run into some gotchas - let me know if you do.
b) This is the 'old' testnet (with genesis hash 0000000224b1593e3ff16a0e3b61285bbc393a39f78c8aa48c456142671f7110)
There is also a newer testnet3 (with genesis hash 000000000933ea01ad0ee984209779baaec3ced90fa3f408719526f8d77f4943) that currently there is no switch for. If you need this I can add it in - just let me know.

You can download the latest classic here.

https://multibit.org/releases/multibit-classic/multibit-classic-0.5.19/

edit

I couldn't get it to sync, it generated a testnet receive address using a multibit.properties configuration file with testOrProductionNetwork=test written in it. I don't know what it needs besides that to sync.