Often times there will be what looks like NULL data in the data, however, it really is No data. (Refer to the blank spaces in the below view).
//the below calculation will place zeroes in the blank spaces above
Partitioning and Addressing:
The lookup function is a table calculation in this formula. By default, it should be computed using either Table (Down) or Table (Across).
If there is data, but it is NULL, it can be formatted using the Format window->(choose measure)->Pane tab->Special Values section->Text text box to show the desired output. Another option is to create a calculated field similar to the following: