Just like frames and other objects, a fontstring needs to either use a template (an existing font object) or have visual properties set in order for you to be able to see it.
Here's a simple, barebones example.
Lua Code:
local f = CreateFrame("Frame")
f:SetPoint("CENTER")
local text = f:CreateFontString(nil, nil, "GameFontNormal")
text:SetPoint("CENTER")
text:SetText("Hello!")
If you are having other problems, we will need to see your code. (We're not psychic.
)