Nested IF statement in Numbers
It looks like you have a lot of extra parenthesis everywhere...    give this a try.

IF(B88="SHADY GROVE METRO","15903 SOMMERVILLE DR",IF(B88="POR TRAIN STATION","CLAY STREET",IF(B88="BWI","7062 ELM ROAD","")))


To provide a bit more detail, for every left paren (, you need a matching right paren ) to close the expression.    Counting what you've shown, you have 5 left parens, and 6 right parens.   This would likely cause an error due to the mismatch.   

Additionally, each expression only needs one left paren to start it, not two.