Look at the policy you have applied to the IDS device in Policy > Intrusion Detection > IPS Policy
Edit the policy and search for 'port scan'
Make sure the attack is enabled and check the threshold value for the attack.
Also the RTTA only displays High and Medium severity alerts, so if it's set to a lower severity use the Historical TA to search for it.
If you still aren't seeing the alerts after following Peter's suggestions verify the threshold on the TCP Port Scan Attack Definition is sensitive enough to detect the scan volume you are sending.
Yes, you're in the right place.
To find out which policy you need to edit go to Policy > My Company > Intrusion Prevention > Policy Manager
You will see a list of your sensors and the policy applied to each interface.
Which attack you need to enable will depend on how you are running your port scan, are you using a software tool and does it have different options for port scans?
As per @jvdavis456 above, there is a threshold set for each attack, you can see it by selecting an attack and double clicking to edit it, look at the settings, you may need to lower this value or increase the amount of scans you're running to meet generate alerts.
After you enable the attack you will need to 'Deploy Changes' to the sensors to make it take effect.
Im running Nmap for scanning
Which i should enable to trigger the alert?
Okay, the test was successful by enabling all Policies for "Port Scan"
Then i ran Nmap on 2 IP host's one is located in DMZ and another one in Corporate Network
The alert triggered for DMZ host not for internal host