Don't even think about using PostgreSQL with Visual Studio without purchasing dotConnect.
The alternatives are npgsql, which doesn't integrate with the Visual Studio data designer, and ODBC, which is slow. And although ODBC allows some use of the Visual Studio data designer, it has trouble with parametrized queries.
Also, don't bother with the free version of dotConnect -- it doesn't include the Visual Studio data designer integration. And, if you can, don't bother with the 30-day trial versions either as it's difficult to convert projects over from the trial version to the registered version.