doesn't show up in the options tab in-game anymore
I think you edited a file which isn't allowed/supported like .mua, .sua or .sga. Only .abp's may be edited. Did you edit the .abp? Can you post the lines that you added?
Although, the steps which Planet Smasher describes are all you need. The mg34 is global available for all units since it's included in the completemotiontree.abp.