When invoking the PlanAhead software from ISE design tools 11.1 through I/O Pin Planning (Pre-Synthesis or Post-Synthesis), in the PlanAhead software (under the File menu), the option to "Import I/O Ports" is seen only when the PlanAhead software is invoked as standalone.
Why is that the case?
This is an issue with Project Navigator and the PlanAhead tool integration.
To work around this issue:
While attempting to open the PlanAhead tool from Project Navigator (Pre-Synthesis and Post-Synthesis), this creates a planAhead folder in the project directory. So, open the PlanAhead standalone and open the "<design_name>.ppr" file from the planAhead folder.
When the project is opened in standalone, the "Import I/O Ports" option is enabled.