The EEG amplifiers have two trigger inputs: a BNC connector for binary TTL triggers, and a 25-pin parallel port for transmitting 8-bit condition codes. When a trigger is received it appears as a vertical green line on the EEG trace, with the numerical trigger code at the bottom (BNC triggers are coded as 1). Some advice on triggering follows.
The parallel port on the ViewPixx display can be controlled through Psychtoolbox. A script is available to demonstrate how to do this.
How to trigger using Eprime
How to trigger from Python
If these methods are not practical, there is one further possibility. If your stimulus computer has accurate control of the sound card, triggers can be sent to the BNC input using an Arduino-based trigger device, designed by Becky Gilbert. One of these is available to borrow from Daniel Baker, or you can build your own for around £30. Further details about this device are available in this blog post.