not able to select columns when completing a measure

Hi everybody,

I'm having an issue with the completion of a new measure. I want to multiply the amount in a certain colum with a certain value. However, if i enter the table name (sales) in the new measure, the column name doesn't appear... Does anybody know what i'm doing wrong? Furthermore, i noticed the table icon in the fields overview, is not a table both a square. Is this related with the problem i am having? If so, can anyone tell me how to solve this? 

Thanks!!!