I knew you had SQL Server background! not that having it is a bad thing, I actually like it pretty much.

Well... welcome to the Oracle world, as pointed by Dave there is not such a thing as attaching a datafile in Oracle.

Let me translate Dave's post.

1-- Make a fresh installation of Oracle binaries.
2-- Do not create a new Database thinking in adding/attaching to it your old datafiles, wait please.
3-- Create a new Database POINTING to your old datafiles; to do this follow the standard procedure of clonning a database.
4-- Startup your database with resetlogs option

Post your session log.