I do not see any problem with my NW 2010 + Win7 64bits, either product or API. Do you have any reproducible sample? Just one thing I would like to mention: Navisworks 2010 provides side by side version with 32bits and 64bits. That means you can install both of them on 64bits.
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This is a core dll which is used in API. It should have been registered when the product is installed. You could try with regsvr32 lcodieD.dll to see if it helps.
In addition, are you using control, automation, or plugin? Can you run the samples provided with the product?