There is nothing that can directly be done about this error: you can’t extend the number of available! No room for a new \dimen.
\
dimen
registers without
extending TeX itself.
e-TeX and Omega both do this, as
does MicroPress Inc’s VTeX.
It’s actually quite practical (with most modern distributions) to use
e-TeX’s extended register set: use package etex (which
comes with e-TeX distributions) and the allocation mechanism is
altered to cope with the larger register set: PicTeX will now load.
If you’re in some situation where you can’t use e-TeX, you need to change
PicTeX; unfortunately PicTeX’s author is no longer active in the
TeX world, so one must resort to patching. There are two solutions
available.
The ConTeXt module m-pictex.tex (for Plain TeX and
variants) or the corresponding LaTeX m-pictex package provide
an ingenious solution to the problem based on hacking the code of
\
newdimen
itself.
Alternatively, Andreas Schrell’s pictexwd and related
packages replace PicTeX with a version that uses 33 fewer
\
dimen
registers; so use pictexwd in place of
pictex (either as a LaTeX package, or as a file to read
into Plain TeX).
And how does one use PicTeX anyway, given that the
manual is so hard to come by?
Fortunately for us all, the MathsPic
system may be used to translate a somewhat different language into
PicTeX commands; and the MathsPic manual is free (and
part of the distribution). MathsPic is available either as
a Basic program for DOS, or as a Perl
program for other systems (including Windows, nowadays).
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=usepictex