A family of Microsoft relational database management systems designed for ease of use.
Hust type the concatenation expressiom in the report's addr text box. There are many ways to to do what you want in an expression, but I think this will do what you want:
=[addr1] & (", " + [addr2])
With text values, the & concatenation operator treats Null values as a zero length string, while + will return Null if either operand is Null.
If you really us a possibility that you can have leading or trailing spaces in the addr fields (Access generally gets rid of them even if they were entered), then use the Trim function:
=Trim([addr1]) & (", " + Trim([addr2]))