Thank you for your help. My problem is not over though, and a new problem has turned up. 

The first steps you described, In NTG-context Digest Vol 51, issue 81, message 8, worked, that is, with sudo.

However, the last four commands, "texexec --make --all", "texexec --make --xtx --all", "luatools --generate" and "context --make", returned the usual error messages (with and without sudo and and issued from any directory along the path "/usr/local/texlive/2008/bin/universal-darwin"):

RBs-Powerbook:/ robert$ sudo texexec --make --all

sudo: texexec: command not found

RBs-Powerbook:/ robert$ sudo texexec --make --xtx --all

sudo: texexec: command not found

RBs-Powerbook:/ robert$ sudo luatools --generate

LuaTools | error unable to identify cnf file

LuaTools | error unable to identify cnf file

LuaTools | no cnf files found (TEXMFCNF may not be set/known)

LuaTools | 

LuaTools | runtime: 0.009 seconds

RBs-Powerbook:/ robert$ sudo context --make

sudo: context: command not found

In addition another problem has turned up: I cannot use Context anymore, with TeXlive-2008 and TeXshop. When typesetting a file I get the following message: 

"/Users/robert/Library/TeXShop/Engines/context.engine: line 3: /usr/texbin/texmfstart: Permission denied"

Line 3 of context.engine reads: texmfstart texexec --pdf --passon="-synctex=1" "$1"

I already changed permissions of file texmfstart.rb to: Owner robert, Read & Write. It didn't help.

When I choose TeXlive-2007 ConTeXt works OK.  

So, what could I do about these two problems (which may be unconnected)?

Thanks in advance.

Robert Blackstone