All AutoCAD entity object (AcadLine, AcadSolid, AcadSurface, AcadCircle, or more generic AcadEntity) as a string property "Material". So, to set a material to an AcadEntity, simply set this property to a valid AcadMaterial's name. Sample code:
Option Explicit
Public Sub test()
Dim mat As AcadMaterial
Dim matName As String
For Each mat In ThisDrawing.Materials
''MsgBox mat.Name
If UCase(mat.Name) = "SITEWORK.PLANTING.GRASS.SHORT" Then
matName = mat.Name
Exit For
End If
Next
If Len(matName) > 0 Then
Dim ent As AcadEntity
For Each ent In ThisDrawing.ModelSpace
If TypeOf ent Is AcadSolid Then
ent.Material = matName
End If
Next
End If
End Sub