I have created an attribute under the Idea object. How do I see it under Portfolios?
Any attribute you create that is not on the Portfolio object itself will not going to show on portfolio list only if you create attribute on portfolio object.
But if you want attribute form idea or from any investment object to show you will be able to accomplish this if after registering that attribute by going to Portfolio Investment object->Attribute Registry to add it from Available to Selected Attributes. After that if you go to Portfolio you will be able to see them on the Investment tab of the portfolio itself.