If you use the overload which takes a string as the first argument you'll need to include the date in canonical string form such as '2015-06-23' or '2015-06-23 19:13:23' enclosed in single quotes like a SQL string literal (within the string argument value itself).  Multiple column:value pairs can be separated by semicolons.  It may be necessary to specify the columns in the order they occur in the index key.  Optional spaces are allowed around the punctuation.

You can experiment with different string argument values in Data Builder in the Data tree node for a table (the outer double-quotes are implied since the text is already a string, not source code).  The Find tab interfaces to that same IVistaDBTable.Find(string, string, bool, bool) method overload--although it's not the greatest representation of what Find does and is particularly unclear in its handling of match failures.  But you can get a sense of what works there before trying it in a test app or your actual app.