At run time, you decide the "bitness" of Integration Services package execution on a 64-bit computer by running the 32-bit or the 64-bit version of the dtexec.exe command prompt utility.
At design time in Business Intelligence Development Studio, you can also specify the "bitness" for running the package from within BIDS. The default setting is 64-bit execution. (This setting is ignored on a 32-bit computer, or if you've only installed 32-bit SQL Server on a 64-bit computer.) But on a 64-bit computer, you may need to specify 32-bit execution when you're debugging a package that depends on a 32-bit-only OLE DB provider, such as the Excel driver.
This True/False setting - Run64BitRuntime - is rather hidden on the properties pages of an Integration Services project, as shown in this screen shot:
Important: This setting applies only at design time in BIDS. This project property has no effect after you deploy your package and start running it by using dtexec or Agent.
For more information on 64-bit concerns in Integration Services, see my earlier blog post: 64-bit Considerations for SQL Server Integration Services.
For more information on other little-known settings for Integration Services projects in BIDS, see this earlier blog post: Profiles, Settings, and More Settings.