Beelzemon is a Digimon from the "Digimon Frontier" series. It is not a Digimon that can be obtained in "Digimon World Dusk".
You can however get Beelzemon Blast Mode, but only through the "Digi-Modify" feature, and only if you have a Digimon that can digivolve into him.
Here's what you need to do:
1. Obtain a Digimon that can Digivolve into Beelzemon Blast Mode: The most common one is "Impmon". You can find it in the "Digi-Farm" by completing quests or by finding "Digimon Eggs" in the field.
2. Train your Impmon to reach the required level and stats to Digivolve into "Beelzemon".
3. Level up your Impmon's "Metal Empire" stat to the maximum level (by using "Metal Empire" related items and fighting enemies with "Metal Empire" resistance).
4. Trigger the "Digi-Modify" option in your Digi-Lab, which allows you to modify certain Digimon.
5. Select "Beelzemon Blast Mode" from the list of options.
You will need a considerable amount of time and resources to achieve this, but the reward is a powerful Digimon!