Longest Path Critical Activities

Dear Sirs.

I am using P6 Ver.7

i have a schedule in that i have an Activity which Duration is 150 days other all activites having max.duration less then 150 days .
This activity Longest path of the schedule

but it is not critical i mean it not showing red Critical line other activitiy which is duration is 90 days shows a red Critical path.

How i can make the max Duration activity become Critical activity.

Thanks & Regards