Assuming the struct to have only double data, this code would get all those indices where the mentioned condition is satisfied -
names = fieldnames(s1) %%//s1 is your input struct
%%// matdata = cell2mat(squeeze(struct2cell(s1)))' %%//' Approach -1
matdata = reshape(struct2array(s1),numel(names),[])' %%//' Approach -2
data1 = matdata(:,[find(strcmp(names,'z_value')) find(strcmp(names,'level'))])
index = find(ismember(data1,[10 5],'rows')) %%// indices where condition is met