You're right: those serial cable components are vital. A remote trigger is nothing more complex than "closing a switch" (in fact you can do it with your finger tip ), something than a standard PC is "electronically" not capable of, at least without a dedicated circuit. In particular, USB signals are totally different animals.
DIY guys use a pre-made USB-to-serial cable (that's not simply "a cable", since it contains a small circuit inside itself) to obtain a simple "on/off" signal at one of the serial "pins", that can be driven through some softwate instruction; then add the necessary circuit (the components you said) to let it become a "switch". Eventually it becomes a "software controlled switch", that's exactly what's needed for a shutter release command.
Luckily, those additional circuits are very simple, and can be realized without a PCB, needing only some soldering skills.
Lacking the skills, I understand that could be scary, but not impossible.
Otherwise, you could find someone near you that's able to help you building it.

Sorry to carry the bad news, but hope to have helped you clarify things a bit.

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