Activations can be see in a DAGNetwork, only the way of write it variesfor the DAG, You need to select the Layer which want to get the activations, and then follow the next lines:
[height1, width1, ~] = size(im);
if isa(net,'DAGNetwork') == 1
featureMap = activations(net, im, 'Relu_conv1');
Map1 = featureMap(:, :, 1);
Map1 = imresize(Map1, [height1, width1]);
repmat(Map1*255,[1 1 3])), 'Size', [1 2]);
the activation feature map is in the position 1 of the matrix, and you can do that with any of the layers.
Hope it helps and is what you wanted. If any question, you may ask me.