Welcome to Spiceworks and its community, a community of IT professionals for IT professionals with a focus on SME.
I'm new here, so please pardon the newbie question
At least you've already started to read our field guides and to apply some of their recommendations. There, you should have seen mentioned the project section with a few words too.
Can anyone tell me how I go about adding projects to my SpiceWorks portfolio?
There are several paths to that project section. All go via your Spiceworks community profile. When your profile offers a link or tab for projects, you may click on it, otherwise you may need to put your profil in edit mode (edit profile button). And when in project section, there is a button new project. When you click on edit, you'll be in mode for editing a new project entry.
Do I have to do one for a customer seeking work here?
It is up to you. When your customers are not IT professionals, your should probably use another resource for targeting your customers. But when your customers are IT professionals too, you may use it also for that purpose. It is primarily intended for showing some domains of your expertise, similar to what is written already in the field guides. And this section is optional.