Are you using DLP 12.5?
If so, remember that DLP Portal Page in Altiris 7.5 SP1 is no longer available, and the only way to do it is using the Software Management Solution from Altiris, or other SMS means.
If deploying using SMS or locally, check the full command used for running the DLP agent. Have a look at the agent_install.bat that comes with the DLP agent. Run this command exactly and it'll work.