When you started Photos for the first time, it created a Photos library from the last photo library you opened. That was probably your Aperture library. To convert your iPhoto Library as well, drag the iphoto Library to the Photos icon on the Dock.
However, Photos cannot merge libraries, like Aperture can. You cannot import the iPhoto Library into your current photo library.
If you want to merge your libraries, do it in Aperture before converting the libraries to Photos. Aperture can import your iPhoto Library into your Aperture library (Aperture 3.3: How to use Aperture to merge iPhoto libraries). Then migrate the merged library to Photos by dragging it onto the Photos icon.
Have you update dot the Yosemite compatible versions of iPhoto and Aperture?
See this User Tip: Updating to Aperture 3.6 After the Release of Photos
And:
Get iPhoto 9.6.1 if you didn't update before OS X 10.10.3