My form captcha is not clickable.

I embedded my form into my Xara website with an IFrame, but for some reason the captcha won't let me type in the box. 

I just figured it out. I had a semi transparent protected object on top of that part of the form so it would not let me click on it.

I ran tests by cloning your form but could not replicate the issue of captcha not allowing me to type letters. On checking the form link you shared I found that their is an html element that is covering the embedded form hence one cannot access the captcha or the submit buttons. I was able to simulate the removal of the element using webtools and I was able to type on captcha and even submit the form. Below is an illustration of the above:

My recommendation would be  to removal of the image in the custom html highlighted above.