Details
Alternate Assembly "New" button is greyed out when the assembly is opened from a Drawing View.
It seems that the Alternate Assembly "New" button is not available when opening any assembly from a Drawing View. Opening the assembly directly solves this problem.
Solution
This behaviour is expected behaviour. If you open an assembly from a drawing view, the alternate assembly tab is inactive. If you open an assembly from a drawing view showing a member of an alternate assembly, you will have access to this one member only.
This is working as designed because there is the link from the drawing view to the assembly showing either the base assembly or a member of an alternate assembly.
Hardware/Software Configuration
Platform: INTL64
OS: windows
OS Version: 1064
Product: SOLID_EDGE
Application: ASSEMBLY
Function: FOA/AP
Ref: 002-8014683