Because OFBiz is distributed in Java source code, it will work on any computer with a System Requirements#Java JDK.
You don't need to worry about architecture, whether Windows, Mac, Linux, BSD, or others. OFBiz can be obtained in at least two ways: by downloading with a web browser, or using Subversion.
Browser download
The easiest way for new users to get OFBiz is to download a current nightly build through your browser. Click on one of the following links, depending on the OFBiz Versions you are interested in, and save the file to your local disk:
Trunk: http://ci.apache.org/projects/ofbiz/snapshots/ofbiz-trunk-current.zip
9.04: http://ci.apache.org/projects/ofbiz/snapshots/ofbiz-rel9.04-current.zip
If these links or files don't work, report it on the OFBiz user mailing list (user@ofbiz.org).
You may also want to try an older build, which is available at http://ci.apache.org/projects/ofbiz/snapshots/.
Unzip the download file in a directory of your choice. I use "/opt/ofbiz", but you may prefer "/usr/local/src/ofbiz" or something else. Windows users often use "c:\ofbiz". If you want to evaluate both versions, you can put them in separate directories, they are fully self-contained. Just run one at a time, and they won't conflict.
Subversion download
Once you get OFBiz up and running, you may want to update to more recent code. This is best done using Subversion, a Code Management System (CMS) that can update only the changed code, without 1) downloading the entire codebase again, or 2) overwriting your local customizations. For further details on using subversion with OFBiz, see https://cwiki.apache.org/OFBADMIN/ofbiz-source-repository-and-access.html.
Prebuilt virtual machines
Sanjeev Gupta has created a VMware image of Ofbiz 11.04 on Ubuntu 12.04 with Oracle JDK. It can be downloaded from http://vsphere.layeredtech.net:8080/4093260325290.
Select and download all files, then use http://www.hjsplit.org/ to combine them.