I agree with Maxim that you need to set the search path for the folder
However, I have noticed the same problem with a .ctb file I received from a consultant. There is something weird going on, I think, with certain .ctb files on a mac system. In my case, the consultant's file was from a Windows system. To solve my problem, I ended up making a copy of the consultant's file (using Command-D on the file in the plot style folder) and then renamed it with the exact same case sensitivity as the original. Mac finder is not case sensitive, but case sensitivity did seem to make a difference in whether Autocad found the duplicated .ctb file. After I copied the original windows file, renamed the old one, and then renamed the new duplicate file with the exact same case sensitivity as the name that Autocad reported as missing, and then restarted Autocad, it found the file it previously reported as missing. Attached are a couple of screenshots showing "before" where Mac Autocad could not find the "raw" windows .ctb file, and then "after" I copied and renamed it. It could be that case sensitivity was a separate problem in my case, and that there is some kind of windows line feed versus mac carriage return issue in the ctb file that duplicating it on the mac corrected. Maybe something like this excercise can help solve your problem.
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John H.