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You basically have two choices: the legacy Blackberry JDE (a stand-alone IDE), or Eclipse and the Blackberry plug-in. I would suggest Eclipse if you have any experience at all with this environment.
The JDE level must be equal to or lower than the target OS. In other words, if you want to develop for OS 4.5, then you will have to compile under JDE 4.5 (not 5.0 or 6.0).
This can present a real challenge, since the newer levels have API's that are not available on the older levels.
We are currently building for 4 different OS levels: 4.2.1, 4.5, 4.7, and 5.0