Mac OS X:Build from source
There are three different ways of installing Gramps on Mac OS X. The linked pages provide step by step instructions.
Installing Gramps as an X11 application is achieved by downloading and installing the packages on which Gramps depends using software called fink. The process is a bit longwinded, but most of the features of Gramps work, and drag and drop and mouse clicks work very well. If you choose this route, you may be able to get some help both from the Gramps community and the fink community. Detailed instructions can be found here: Mac OS X fink.
Unfortunately, due to The Great Gnome Update, fink does not currently support any version of gramps beyond about 2.2.6. As of January 2008, the estimated date for release of an update that would support later versions of gramps is 'soon'.
Note that fink is designed to support binary distributions and automatic building of packages, but the latest version of gramps that is 'fully' supported by fink is about version 1 of gramps (i.e. very old), so after you have installed the dependencies, you have to install gramps 'manually'.
MacPorts is essentially a similar type of program to fink. It is not as slick as fink, but is (currently) more up-to-date.
See Mac OS X MacPorts on the installation process.
Native Mac OS X
You can install it as a native Mac OS X application which means that Gramps windows have the normal appearance of Macintosh Windows. This choice has fewer dependencies on installing other components. However, it has the disadvantage that drag and drop does not work, and there are issues with mouse clicks going missing, which make it much more difficult to enter information like sources which apply to many different people in your family tree. This is also in some ways a less mainstream approach so it may be harder to find help if you get into difficulties.
Am I ready for X11?
Installing with either fink or MacPorts is simpler that installing as a native application, but the Gramps windows have an X11 appearance. To see whether you can get use dto this appearance, you could download and install OpenOffice which is very easy to do and will show you what an X11 application looks like. Installing OpenOffice will not be wasted, because you will need it to be able to open reports from Gramps.
Another option is to download NeoOffice, which is a port of OpenOffice using Java so that it looks like a native Mac OS X application.
Are there any other options?
(1) Virtual Server
Yet another option is to install a virtual server on your Mac (like VMware Fusion), create a virtual machine running Linux, and install Gramps in there. A Gnome based distro like Ubuntu would be a good choice for the Linux system.
(2) After already having all the dependencies installed
If you have all of the dependent libraries installed already, this should be straightforward. Just download the .tar.gz file, unpack it, and follow the INSTALL instructions.