The easiest way to find it is to open the solution explorer tab, and
press the button at the top of tab that shows all files. Then press
the [+] button next to the desired form to expand the file choices.
Double click the .Designer.Vb file and you should be set.
The new 2005 designer takes advantage of the new partial classes featuires
of the 2.0 framework so the generated code is kept in a separate file as
rowe says.