TeXmacs is available in
apt-get install texmacs
As usual, you may obtain further information about the TeXmacs package (dependencies on other packages etc.) by executing the command
apt-cache show texmacs
Alternatively, you may obtain the package information and the package itself from the following web pages:
If you are running the "unstable" distribution, you can get TeXmacs from "experimental" (by including the corresponding line in your /etc/apt/sources.list) which is rather up-to-date. Versions in "stable", "testing", and "unstable" are way too old. Unfortunately, the package is orphaned. Any volunteers?