It is quite an easy process to create a custom truss in ArchiCAD, it only take lines to create, just follow a few easy steps outlined below.
First go into section view
Select line in the toolbox and draw a truss on a desired location
Take note that the lines act as an axis hence the assigned thickness of the truss projects from the centre.
After outlining the truss drawing, you may highlight a multiple of lines or a single line and go to Design > Design Extras > Truss Maker > Create Truss as in screenshot 1 below
Truss Maker settings dialog box will pop up, assign attributes and truss profiles as in screenshot 2 below
After assigning attributes and truss profiles click Ok, a save truss dialog will appear as screenshot 3 below, type in the name of the truss and click Ok, it will be saved in the embedded library and created on the floor plan as per screenshot 4 below, click Ok.
For further details and illustration please log into the following link: http://youtu.be/aar-7dEwN0Y