There are two ways to model lumped mass 

1.Model lumped mass in Nodes of the structure. (Usually used in beam problems and can be used in others too. The lumped mass in kg, or grams can be directly entered to the space provided in window.)

2. Model Lumped mass in selected areas or volumes. ( The region where we need to model lumped mass can be selected and enter the mass.)

Step 1. In module select INTERACTIONS

Step 2. In menu 

              SPECIAL -> INERTIA -> CREATE

Step 3. Create inertia window will come. Now you can see the options point mass/Inertia, Nonstructural mass. Choose which one is suitable for your problem.

Step 4. Select the regions where you need to model the lumped mass

Let me know if you come across any other problem while doing this.

All the best

Dear Sharon Balasubramanian

Thank you so much for your answer.

If I define point mass in property module, Is it different in comparison to defining in interaction module ?

Which one of way that you say is better?

Thanks again,

Azadeh

It depends on your problem.

In frequency analysis If you defined in property module by creating a new material by changing the density it will give the same result since the software takes the mass only. But that't not the right way to assign lumped mass. Because while in property module we are defining a material with it's actual material property data collected from some journals or any other authenticated documents. Even if it gives the same result, It's not preferred to do so. If you are just  concerning about answer it's OK.

 If you are working for a paper, we will tabulate the property of material at last. It's always better to right original property values of material and lumped mass as separate.

 Hoping that this will help you. if it helps give me a support vote for answer.