How can I used shortcode for a button?

Guys,

I am looing for a button plugin which accepts shortcode.
I am using this theme http://wp.color-theme.com/streak/ theme has few shortcode for button.

like: [button style='2' caption='Primary' link='button url'][/button]

Here the caption for the button is Primary.
in place of link I want to insert a [ajax_login] shortcode.
Could you please help me acheive that?

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Basically I want this button to work like this.

I want to insert this "Join Us" button in my pages.
On tap of it there should be a login form which pops up.

Could you please tell me how can I do this?

this is my site: http://wowdesigns.in/projects/nesties/

Thanks.