There is no option for Arrangements on the Assemblies drop-down menu.
Solution
It is likely that the Assemblies Application has not be enabled. Select the Application tab on the NX Ribbon and check to see if the Assemblies icon, within the Design group, is highlighted.
Notes and References
Hardware/Software Configuration
Platform: INTL64
OS: window
OS Version: 1064
Product: NX
Application: ASSEMBLIES
Version: V1847
Function: ASSEMBLY_MODEL
Ref: 001-7204802