Hi there should be a little lever on the bobbin case itself pull it out and the case should come out. I used to sew on my Mom's 301 that was 30 years ago so i am pretty sure thats how you do it. I know you have to put the presser foot down. than turn the wheel to lower the needle and bring it up and pull and the bobbin thread should come up.
I sure made the biggest mistake when I sold my moms 301 after she died because I had a new singer and never thought I would use the 301. Mistake. good luck with yours.
Marg