これバグでしょ？
aadisplay なんてプログラム知らんし。

(funcall navi2ch-article-view-aa-function "hello")
=>
Debugger entered--Lisp error: (file-error "Searching for program" "no such file or directory" "aadisplay")
call-process("aadisplay" nil nil nil "/tmp/tmp30095MwG")
navi2ch-article-call-aadisplay("hello")
funcall(navi2ch-article-call-aadisplay "hello")
eval((funcall navi2ch-article-view-aa-function "hello"))
eval-expression((funcall navi2ch-article-view-aa-function "hello") nil)
call-interactively(eval-expression nil nil)

(defcustom navi2ch-article-aadisplay-program
(if (eq window-system 'w32)
"notepad"
"aadisplay")
...)