When usign NX 10 Motion Simulation you can use the option to Fix a part to use as a ground. To Fix the part in space, in spanish is translated Fix as Repair (Reparar). So to anchor a part in the space you must click on Reparar.
Please report this issue to GTAC at http://www.plm.automation.siemens.com/en_us/support/gtac/index.shtml
so that it can be corrected.