Hey mgallant,
Well, looking at the SOLAR Entry Schedule, students in Level 2 of the Faculty of Humanities can enter SOLAR on Wednesday (June 25), while those in the Faculty of Social Sciences can enter on Thursday (June 26). My advice to you would be to try and see if you are able to enter SOLAR on Wednesday (as a Humanities student). If not, then Thursday (as a Social Sciences student) it is! My guess would be that you are still in the Faculty of Humanities and you should be able to enter on Wednesday.
In terms of choosing the major, as soon as you enter SOLAR, you will be asked to pick your program of choice, which would be Honours Psychology for you. Then, you will be taken to the Course Selection webpage, where you will be able to select your courses.
Good luck with course selection!