Hi Abhinav,
Instead of previewing from Portal Content, I would suggest to open the Role where the iView is added and do the personalization from there.
Confirm if you are using Web Dynpro Java Applications and not Web Dynpro ABAP applications.
1. Open the Role
2. Navigate to the iview you want to personalize
3. Open the iview and preview
4. Ctrl + Right Click to do the personalization.
Hope this helps.
Cheers-
Pramod