You have to setup a variable and assign it a value equal to the path of the file you wish to export to. Then when you setup the connection manager to Excel you go into the properties and asign the value of the variable as an expression in the connection string value. There are quite a few articles and forum posts on-line that demonstrate this but here is one of them.

http://www.sqlservercentral.com/Forums/Topic530390-147-1.aspx