Welcome to the forum.
I'll answer here since your other post is the same question. This is a user forum, not Cakewalk support, so there isn't someone here 24 hours a day to answer questions; sometimes you have to wait awhile for a reply.
Without a MIDI keyboard or controller, you have to enter the notes one by one in either the Staff view or Piano Roll view (PRV), as you know.
To use SoundCenter, you click Insert at the top and select your synth from the list. If you already have a MIDI track, you can uncheck the Create MIDI Source track box. If you're starting from scratch, leave that box checked and MC will insert a MIDI track with the output already set to the synth and it will insert the synth. Set the synth's output to a bus (preferred) or your sound card.
In the SC GUI (the window that should pop up with the synth's properties), select your instrument and the specific patch you want to try.
Now click on the MIDI track and go to Views and choose either Staff or PRV.
Select a note resolution in the box at the top and the Draw tool, then start clicking notes into the PRV or staff. You should hear the notes play as you click them in.
There is more info in the Help files and there are tons of videos on the internet covering this subject. Many are for Cakewalk's X1 and X2 programs, but the method of selecting synths, and entering notes
should be identical.
Good luck!