You are new to the forum - how new are you to ArchiCAD?
Try to load from file dialog box instead of object dialog.
Is the object actually in a loaded library?
If on PC, is the extension correct?
Did you try the old "drag the object into the plan view" method?
Is this object perhaps mistakenly saved as a window, door or lamp, in which case it won't be accessible from the object dialog. What is your ArchiCAD version - courteous to post it since object saving changed with the newer version.
If this doesn't work, call me at 604-266-7322 tonite or tomorrow and I will brainstorm with you.
Dwight Atkinson