When you open manage budget categories, have you gone to the Lower left options button and chosen assign categories?
Please use the actual names of the things you are clicking on. If you go to Tools .> Category List and look up your SSA Gross category, is the Type Income? If not, edit the Category to fix this. Is the Hidden box checked? If so, un-check it.What is its Group?
Then go to the Planning > Budgets page, click on Manage Budget Categories, and find the appropriate Group. Is it still not there?
BTW, not clear to me how you are using both a gross SSA category and a net one.
Both SSA gross and SSA net should not be budgeted. If SSA net is budgeted, then do not budget for Medicare premiums and SSA income tax deductions. If SSA gross is budgeted, then Medicare premiums and SSA income tax deductions should also be budgeted.
I use the SSA gross option, not just for budgeting purposes but also for accuracy in the Tax Reports and Tax Planner.