If you go to Colby's Library Catalog, do a title search for the particular journal the professor wants you to use.
Whether you find it available electronically or in paper, you want to look in one of the most recent issues. You want to look for something like:
This information is usually on the cover (front or back) or in the first few pages of a paper journal issue. In an electronic journal, it might be in the header or in a side column. You just have to look around, but it should be fairly prominent because they want potential authors to see it.