I've tried to use HTC Tattoo as a modem on a Windows PC and it worked well, but on a Mac it doesn't work.
Some trick or app that i should be aware?
My HTC Tattoo uses the default 1.6 ROM with Sense.
Hopefully the Mac version of PdaNet for Android will do this for you. There's a trial period, after which you need to buy it if you want to browse secure (https:// …) sites. Signing into Gmail requires HTTPS, for example.