My macbook pro (2011) updated to OS X 10.10.5 and Photo was installed. Most of my older Photo events are missing
I can open iPhoto on my Mac with Yosemite, so it might be as simple as upgrading.  I have version 9.6.1. 

Photos for me does not show a 'iphotos event' tab, rather, the images from iPhoto are simply in Photos. 

If you think the iPhoto images did not get imported into Photos app, then do the following: Open the "Pictures" folder, and look for the old iPhoto Library icon. Right click and choose 'Show Package Contents'. Within this is a folder called 'Masters'. Open this and you will see all your photos.

Now, do the same with the new Photos Library. 'Show Package Content' on it, and you will see a 'Masters' folder. You will notice that this Masters folder contains the same images, including your old images. If it does not, something happened on the import during install.

You can simply select all the images within the iPhoto 'Masters' folder and drag them into Photos, which will add all those photos to Photos app. It will duplicate those images, which the Photos app install does not do.