To change the name of the source data file at runtime you can use an ActiveX Script Task. In the following example the new filename is read from a global variable called ImportFilename. Please ensure you change the bulk insert task's name value ("DTSTask_DTSBulkInsertTask_1") to match that in your package.

' Pkg 231
Option Explicit

Function Main()

	Dim oPkg, oBulkInsert
	Set oPkg = DTSGlobalVariables.Parent
	Set oBulkInsert = oPkg.Tasks("DTSTask_DTSBulkInsertTask_1").CustomTask

	oBulkInsert.DataFile = DTSGlobalVariables("ImportFilename").Value

	Set oBulkInsert = Nothing
	Set oPkg = Nothing

	Main = DTSTaskExecResult_Success
End Function

If you are using SQL Server 2000 then a similar approach can be taken using Dynamic Properties Task.