Yesterday morning, I studied English on Livemocha. In the afternoon, I was working.....
Sorry, I'm not sure how to correct the last part of your sentence. I don't know what a "scout group" is.
Yesterday morning I was studying...
Yesterday morning I studied...
Either one is correct. It would be more common to use "i was studying" when describing something else that happened at that time. "I was studying English on Livemocha when my friend called.
at Livemocha
on Livemocha os dois sao iqual, "on" might be more common in the EUA.
In the afternoon, I was working in a scouts group.
-It might be better to also describe what the scout group was doing - "working in a scout group on a service project". Your sentence implies you are a member of the group, you are "in" the group. If you had written "I was working with.." it would imply your job was to lead the group.