The program 'curl' is currently not installed

Answers to the different updates

First question

You need the command

sudo apt-get install curl

The install is missing in the command you use: sudo apt-get curl won't work because apt-get does not have a command curl.

First update

Also you want to run update first:

sudo apt-get update

And you might want to consider using this command:

sudo apt-get install libcurl3 php5-curl

It might also relate to the version of php you use (php5-common?) See here.

Second update

Or, to make it easier just use the command wget https://install.meteor.com | sh. Curl was used on earlier versions of ubuntu. It is not really an essential tool, but some tutorials use it indeed.