1] Right-Click on XNET Databases and browse to your .dbc file
2] Right-Click on NI-XNET -> CAN and select "Add CAN Port"
3] Confirm Interface Name, Baud Rate, etc. in CAN Port Specifications
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If I misunderstood your question, and you are actually trying to look at the .dbc file to edit it or make changes, you can use Start -> Programs -> NI-XNET Database Editor.
Most .dbc files can also be opened as .txt in Excel