Angela - I think you might be looking for the Formspring Wordpress widget? That's the one with the "Ask Any Question" box, as far as I can tell, and is probably what you've been seeing in the sidebars of other peoples' blogs. The link contains all installation instructions, if so.
Thanks Emily. The Formspring Wordpress widget you've linked to is applicable to Wordpress.org, but my blog is built on Wordpress.com, and apparently that makes all the difference. It's all very confusing.
Guys, checking out Google results for things like this can be really worthwhile. I searched for "formspring wordpress help" and found this thread on a much more relevant forum, for example, where a little more searching would probably find you a whole bunch of solutions.
If you read to the end of that thread (which I'd already found), you will see that a solution is not offered. Similarly, searching 'Formspring' in Wordpress offers a number of threads about the subject, but no solutions. The matter in each case is not followed up either by Wordpress or by the person asking the question. Wordpress's answer is to use their contact form, but I want the Formspring box because it's better imho! (and I've seen it done on other Wordpress blogs).
It's not that we don't have tech geniuses here but it would be a lot quicker for you if you head to places online where your specific issues have been/are being discussed already. (Putting your exact query into Google is surprisingly helpful. Even "help I can't tie my shoelaces" yields incredibly useful and specific advice. )
To be fair, I did a lot of Googling around the subject before I posted here for help, and I only did so because I remembered DeeDee having the same problem and I wondered if she'd sorted it and maybe she could tell me how.
I also had already emailed Formspring (who told me they didn't know how it was done, and told me to contact Wordpress) and Wordpress, whose response was as follows: 'Unfortunately, we cannot allow that type of code on WordPress.com for security reasons.' I have responded, telling them that I've seen it done on other Wordpress sites, so there must be some way of doing it. I am awaiting their response. In the meantime, I think I've figured out the solution for myself - it's the difference between the .org and the .com, as I've mentioned above. I am not yet 100% sure, but it appears that plugins are not 'allowed' on .com, but that they are on .org. I await Wordpress's clarification of this.
DeeDee, if you are still having problems, I will let you know what the response from Wordpress is as soon as I hear. It may be that the only solution is to build your blog on Wordpress.org as opposed to Wordpress.com.