To install the DSA manually simply locate your Migration Manager downloaded package of which you used to install the program. Within those extracted files are other installer packages including the DSA.msi.
The MSI is also available from the MMAD console machine, the default path being:
C:\Program Files (x86)\Quest Software\Migration Manager\Common\BIN\DeployDistr\DSA.msi
Open your extracted files directory and enter the "CD" folder. The "DSA" folder beneath that will be present. Copy this folder to your server and install the DSA by selecting the DSA.msi.
Return to the tool and attempt to install the DSA agent on the server again, remotely, via the Agent Manager. The agent manager will register the DSA location instance within Migration Manager project database, but may report the agent already exists. The new agent will now be available for selection within Migration Manager for AD.