The supplier probably sent you a pair of foam donuts, these will probably have to be trimmed in length, glued to the hubs and then trued.
Check with your supplier, he may offer this service for a small fee.
Don't know who sells Ortman urathane tyres in the states but am that they would have one to fit your hubs, in the UK you can try RS Slotracing.com.
For foams look at the harder wearing Wonder tyres by Alpha [I think], these would give you good grip and last much longer than the softer foams.
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