On the EOS 450D, mounting something in the hot shoe generally triggers the hot-shoe detect switch, which disables the pop-up flash. So with the GPS unit mounted directly in the shoe, you typically cannot use the built-in flash at the same time.

A possible workaround is to move the GPS off the camera’s hot shoe using a hot-shoe adapter and sync cord, or use an optical-slave setup. Since a geotagger like this should only need the basic flash trigger signal—not brand-specific TTL communication—an adapter may work. One answer notes that a sync cord may allow the pop-up flash to remain usable, but this should be tested before buying.

Physically modifying the accessory to avoid the detect switch was also suggested, but that’s not recommended.