1994 Other Isuzu Models Different wheel installment

Other Category problem
1994 Other Isuzu Models 4 cyl Two Wheel Drive Manual 104k miles
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when i try to install new rims on to my truck there is a black think in the middle of the "router"?
well yea and my rims wont fit with that
black thing there..
so yea what is that black spacer looking
thing andhow do i get my rims on there?
thank you very very much.
-brandon