[NTG-context] ConTeXt installation problem using i-installer(v 2.85)

Dear all, Yesterday, I updated ConTeXt using i-installer(V 2.84). It worked fine with XeTeX too. I am able to make presentation using ConTeXt + XeTeX which was not possible before. Today I tried to update ConTeXt in Mac located in school. At this time, I used the upgraded i-installer(V 2.85). But it didn't ask me the permission(no Authenticate window). So it couldn't have a write permission in the directory /usr/local/teTeX/. I got the warning/ error message, "/usr/local/teTeX/share/texmf.local is not writablle. I cannot change a nonwritable TeX. Bailing out...". Hence I couldn't install ConTeXt. What do I have to do to complete the installation? Thank you. Dalyoung