Extrude one half of the model and snap the verts that are in the mid to the center. When completed mirror, merge verts and border/edges. It should to be in the mesh menu. Before deleting the history under the transformation panel to the right, click mirror, and set the threshold to 0.001 or 0.01 and delete history. For the…
In order for merging to work you need to have them pretty much on top of each other. What you should do is select all verts down the middle seam and scale them in on that axis. This will close the seam you have and make sure the verts are on top of each other. Then the merge vertices tool with work.
It's best to scrub the scale handle in a few times to get them close. Merge vertices (by default) will always merge if only two verts are selected. The default merge tolerance is too high for my tastes, so echoing what BARDLER says. Smooth mesh preview will let you know pretty quick if there's any issues in your topology,…