Your code
sscanf(get(handles.edit6,'string'), '%s')
has the same effect as
get(handles.edit6, 'string')
unless the contents of the edit box have multiple parts separated by spaces (in which case your code pulls out only the first one.)
In order to get 2.0*10^(-8) you would need to convert to numeric form and then convert the numeric form to string using a different format.
If you convert to numeric form and store numbers into UserData.celkovamatica instead of strings, then you could set ColumnFormat to 'short g' to get 2e-08 as the output.
If that is not acceptable, then you will need to store strings (like you do now) and format it yourself, such as
s = sprintf( '%.1e', str2double(get(handles.edit6,'string')));
and then do string manipulation on the '2.0e-08' that results
UserData.celkovamatica{1,1} = regexp(s, {'e', '$'}, {'*10^(', ')'})
