Way A:
You buy or select an addon. Login into your CMS, goto packages and hit install.

Way B:
You (buy and) hit download on the concrete5.org addons site. Download the addon and unpack the contents into your /packages folder inside your c5 installation.
Then you login into your CMS, goto packages and hit install.

You can use an addon on as many sites as you want as long as the license of that addon allows it.

cheers
Fernandos

"You can use an addon on as many sites as you want as long as the license of that addon allows it."
actually i think there aren't any paid addons in the marketplace that allow unlimited installs. the standard concrete5 requires purchasing one license per site.

Why you need to use the many sites?

If we're doing a job, a customer can make a claim to the necessary consideration.

If you're doing your hobby, it depends on how good bypassing the money.

If likely to be paying so much money
If copies are free to develop their own.

kino: i'm having a bit of a hard time understanding your last post, but the main reason for charging for each install of an addon is because of support. it's a bit unfair if someone using an addon for their personal site pays the normal price, and then a design shop can use the same addon a thousand times for all the sites they develop, and only have to pay once for the addon while getting the same level of support. i don't mind sending out freebees to the occasional non-profit or something, but when packages are typically just $15-50 dollars i don't see why fulltime developers and design shops can't include that cost on the invoice to the client. especially since on larger sites the cost of addons can be such a tiny percentage of the totally site cost.