Application loader won't sign in

Running an old iMac. Got it second hand.

First thing I did was to create a new Admin user and delete the old one.

After that I installed the newest available OSX - El Capitan 10.11.3

However I still have two issues that I cannot seem to solve despite plenty of googling.

1. Application loader will not allow me to sign in - it states that my ID or password is incorrent.

a. I know my details are correct as I can sign in through a browser on my other computer without any issue.

b. All the solutions I have found recommend using the application menu to run Window/run setup wizard or run Window/Sign in as : These options are not available to me; Window/Sign in as is grayed out.

c. Is there a manual way to delete the application's memory of its old login? Perhaps through keychain?

2. The second issue I am having is that I cannot update iPhoto; it states the app was purchased by someone else

Many thanks if anyone can give me any new options I can try.