Hi,
If you want to use the source code with pgsql on windows I recommend you get the code from svn trunk. There were some bugs in the upgrade scripts for pgsql reported recently that are fixed in svn trunk but not in the previous release.
To work with the mojoportal source code you must have VS 2005 SP1 or VS 2008. mojoPortal uses the precompiled model not the inline code model so you can't just edit code on the server with a text editor and in fact you should not in my opinion deploy c# source code on a production machine. Instead you build it in VS then package the deployment files using the free UnleashIt tool.
Also when working with the source code you must build the solution with VS before it can work and the procedure for using a different data layer is not the same as with packaged release files. You change the project reference from mojoPortal.Business project, first remove reference to mojoPortal.Data.MSSQL, then add reference to mojoPortal.Data.pgsql, next rebuild solution. More info here: http://www.mojoportal.com/settingadatalayerreferenceinvisualstudio.aspx
Also with the latest svn code you must set this in Web.config or user.config
<add key="SubSonicProvider" value="subsonicPostgreSql" />
Hope it helps,
Joe