Hi Deepti,
If you wanted to know the difference between pdm_load & pdm_userload go to command prompt of service desk server and run pdm_user load -H / pdm_userload -H , you will find the list of switches awailable. As mentioned earlier pdm_load -i will be used.
If you have a huge list of contacts in csv file which needs to be imported then contact CA Services should be able to help you.
With regards, Vinod.