Hi Brittney Mitchell - It looks like the issue here is that in Jive you're only allowed to have four custom navigation menu items. This explains why you're not able to add any new ones in your custom theme:
From our product docs:
If you want to change the placement of the buttons in the navigation bar, just click and drag them into the desired location. You can also add a new button to the nav menu by clicking the plus sign
up to a maximum of four custom buttons.
If you only had three then you would see the + symbol on the far right to add another one, like this:
Let me know if this helps answer your question
Very helpful, David. Thank you!