According to
Blizzard's UI code, the name of the XP bar is "MainMenuExpBar", and nothing (at least, nothing else defined in that file) is parented to it, or anchored relative to it, so this should work:
Code:
MainMenuExpBar:ClearAllPoints()
MainMenuExpBar:SetPoint("CENTER", UIParent, "CENTER", 0, 0)
For good measure, you can also explicitly set the position relative to UIParent for the other frames:
Code:
MultiBarBottomRight:ClearAllPoints()
MultiBarBottomRight:SetPoint("BOTTOM", UIParent, "BOTTOM", -254, 100)
MainMenuBar:ClearAllPoints()
MainMenuBar:SetPoint("BOTTOMRIGHT", UIParent, "BOTTOMRIGHT", 50, 0)