Try:
set myFolder to POSIX path of (choose folder)
do shell script "find " & quoted form of myFolder & " \\! -name \".*\" -type f -print0 | xargs -0 chflags nohidden"
or
set myFolder to POSIX path of (choose folder)
do shell script "find " & quoted form of myFolder & " \\! -name \".*\" -type f -print0 | xargs -0 chflags hidden"