Microsoft Project does not exist on MacOsX ! (In February 2012)
So, here are all the solutions I have tested to get around this issue. I choosed Gantter.
- Gantter : http://gantter.com/
- Freeware
 - Online in browser mode
 - Works with Google Docs
 - Probably one of the best tools at the moment. This is the one I choosed.
 
- xPlan : http://www.adnx.com/i/fr/apps/xplan4mac
- Not free, there is a TRIAL version.
 - An interesting client, but needs some fixings, adjustments and maturity.
 
- Merlin 2 : http://projectwizards.net/en/merlin/
- Not free, there is a TRIAL version.
 - Quite a good client but not free.
 
- Project Manager : http://www.projectmanager.com/
- FREE 30-day trial
 - Fully functional but quite expensive.
 
- Open Project : http://sourceforge.net/projects/openproj/
- Open source project.
 
- Gantt Project : http://www.ganttproject.biz/
- Fast Track Schedule : http://www.cesyam.fr/fts_html/fts.html
- Rational Plan : www.rationalplan.com
Cheers,
Alexandre