Each KeypadLinc button uses a Group number based on a hardware convention. In 6 button mode, the large ON and OFF buttons use Group 1. The 4 smaller buttons, A, B, C, D use Group 3, 4, 5, 6 respectively. Button C uses Group 5 (unit 05).
There is nothing special about creating a Scene with a KeypadLinc. Since you have an EZBridge I would download the free SimpleHomeNet Utility Suite and use it to display the link database in the KeypadLinc. It can take many minutes to work with a link database depending on the current content because the database is accessed 1 byte at a time, with each link record being 8 bytes long. Running the Utility will show what is in the KeypadLinc link database as well as determining if there are communication problems with the device. It could be on the opposite 120V leg as the EZBridge, the EZBridge Insteon signals could be attenuated by the power supplies of a PC and/or UPS that is generally connected close to the EZridge PLM. If the Utility can display the KeypadLinc link database consistently without errors, the EZBridge should be able to do the same.