To change this you'll need to go into your adobe account which is providing the font. Web fonts are made as small as possible and therefore don't export all the characters by default.
this is because of the problem I've written about in previous posts in this thread. but the macrons will probably not go into the alda font. This should make everything go all 'alda'. so if you add this to your custom css you might find it works (I've only done this for the normal font weight, but it seems to However, with a bit of trial and error it does seem like you can overwrite the particular subset it brings down by modifying the font family declaration.
You can't directly alter the subsets that typekit download as they are done through Squarespace. The Omnes Pro font squarespace provide is from typekit and the particular unicode character you're looking for isn't in the subset provided so the browser is falling back to whatever default font it does have that character in. Those subsets are usually conditionally rendered depending on whether a particular character is found in the page. However, font files are quite large and as such when you use webfonts they are usually split up into smaller subsets (as most of the characters aren't used). The code you see on your computer is from the locally installed OpenType font which has the full set of characters.