Are you using VS to write code? if yes, did you notice that after you enter "." following MapApplication, there is no interllisense prompt for you to choose a property called "Aliases"?
You should use HostMapApplicationServices' Application property (which is an instance of MapApplication) to reach Aliases:
Dim aliaseList As Aliases = Autodesk.Gis.Map.HostMapApplicationServices.Application.Aliases
HTH