OK, here is what they told me. You could put your name, address, telephone number, store name, purchace date on a piece of 3x5 card, put one extra line explaining rebate coupon is missing from the package, send this along with boxtop with barcode, copy of receipt, to the address below, and they will send you MIR check.
Fro Microsoft SiderWinder FreeStyle Pro:
Microsoft SiderWinder FreeStyle Pro
Promotion number 497-00-664
PO Box 1128
Ridgely, MD 21681
For Micrsoft Natural Keyboard Eltie/Wheel Mouse combo:
Micrsoft Natural Keyboard Eltie/Wheel Mouse Package Set
Promotion number 497-00-699
PO Box 1176
Ridgely, MD 21681