Why is my theme directory unchangeable?

[CountryCrone]

SMF Version: SMF 1.0.5
Hello
  This is the first experience I have had concerning forums on my site.  I installed it from Fantastico and I have had so much fun learning about it.
My problem is that I wanted to change the look of my forum and I downloaded one from the Simple Macines website that I really love.  I downloaded it first and then uploaded it to my server.  Then I went back into the forum Admin Board and tried to change it.  This is what it said:

Warning - you cannot create or install a new theme as your themes directory is not currently writable!

I went back to the SM site to find answers and have been looking for hours and I can't find any answers.

Can someone please help me?

[xenovanis]

Make sure your themedirectory is writable. (chmod 777)

How do I use chmod?