[R] manual installation errors; install.package, R CMD

[message]

Readers,

As root user, from the command:

install.packages()

A dialogue window appears to select packages of interest. What to do if 
a specific packages exists on the cran page 
(http://cran.r-project.org/web/packages/available_packages_by_name.html), 
but not in the dialogue window of a repository?

Sometimes, a dependent package is reported as being un-available, e.g.:

> install.packages("mclust")
Warning message:
In getDependencies(pkgs, dependencies, available, lib) :
   package ‘mclust’ is not available

Alternatively, tried the command 'R CMD':

R CMD INSTALL 
http://cran.r-project.org/src/contrib/ChemoSpec_1.61-3.tar.gz
Warning: invalid package 
‘http://cran.r-project.org/src/contrib/ChemoSpec_1.61-3.tar.gz’
Error: ERROR: no packages specified

The FAQ web page 
(http://cran.r-project.org/doc/FAQ/R-FAQ.html#How-can-add_002don-packages-be-installed_003f) 
states that the above syntax should be accepted.

What is my mistake please?