This can be done by creating a mark set based on an assignee of interest. To do this, firstly type the assignee into the "Current Assignee" text box from the "Search" area of the landscape and click on "Search".
Once you have done this, the patents that have this assignee will be highlighted with the landscape. You can create a mark set for these patents by selecting "Create Mark Set". You also have the option of adding these patents to a Workspace or downloading them.