Placing a Device in EAGLE
You can only select different packages for a device, when the author of the library provided these. Take for example the ATMEGA32U2 in the adafruit library. This device has only one package (TQFP32-08). The ATTINY10 device on the other side provides two packages (SOT23-6 and UDFN-8).

When you find a device that has not the package you need, you can create your own. Create a new library and reuse the symbol of the device. Various package formats are in the ref-packages.lbr library. The final thing to do now is to connect the right pins of the symbol with the pins of the package.