Well I can't speak for the folks at Cheat Happens on their specific methods but having plenty of first hand experience I can make logical deductions as to how anyone would go about it... Also, keep in mind that their trainers typically use pointer editing rather than direct value modification, but that's a lesson for another time.
First I recommend a more powerful hex editor, Cheat Engine is a much more useful and versatile tool.
Second, always know your target, often in games with ever increasing values like the resources in COH, they are stored as floats (decimal values) rather than integers (whole numbers).
To search for them is a bit more tricky than an exact integer value because there are tiny fractions out past the decimal point. With Cheat Engine, you can choose to 'truncate' your search, which basically means you're just chopping off any digits past the ones you specifically typed in to search for so that only the significant digits that you wanted come up in the search results. What this means is that if the value is technically 256.123, you can just search for 256 and all values of that with any additional decimal value will be found and you can comb through them in successive searches.
Another way, although much more tedious is a ranged search, so if you have a value you know is between 10 and 11, you can of course set the low range to 10, and high to 11, and all values in there will be grabbed, but this is typically a much slower and less efficient search than just using a more efficient tool, particularly when the result would be the same.
Poke around on the internet for many more lessons on hex editing, it's really pretty simple to pick up, and the more you do it, the more advanced tinkering you can do. Best of all, with modern computer operating systems, the most you're likely to do if you mess up is crash the game, in which case just start up again and don't do what you just did again