This teacup subcommand fetches and installs a package for you.
It is important to note that there is a syntactical distinction between installing a package (aka an extension), and an application.
For example:
$ teacup install sqlite3
is what you would type to install a package.
$ teacup install tkcon -is application
is what you would type to install an application like tkcon.
SPECIAL NOTE
If you are wanting to use teacup to install tcllib, it too takes the special syntax:
$ teacup install tcllib -is profile
This is because tcllib is NOT a package - it is a collection of packages.
If you are wanting to install the batteries included set of packages that came with ActiveTcl 8.4 (and to some degree, earlier), then you would use
$ teacup install ActiveState::ActiveTcl -is profile
Now, some people may find they need to run teacup timeout to lengthen the amount of time teacup waits for completion. And others may need to look into teacup's -proxy argument.