[plt-scheme] 399: how to get the internal helpdesk browser?

I just installed 399 from source. I downloaded the
plt-3.99.0.24-src-unix.tgz, ran configure --prefix=/opt/plt and make
(no problems), then make install, which died because there was no doc
directory. So I downloaded the PLT Scheme Full Unix Sources
(plt-clean-tree.tgz), copied the doc and man directories from there to
the plt-399 directory, and make install worked fine. The problem now
is, both the "help desk" link from PLT and plt-help from the command
line simply open up the webpage in gvim, and I can see no option to
invoke the internal browser. The best I can find in the preferences is
to make it open in a browser instead. How do I get the internal help
browser back?

martin