Lets make it clear from the start. There is NO SUCH thing as a code that will increase your search rankings! It doen't exist.
Now to answer your question. Most directories require you to put a certain code/link on your website so before they agree to put yours. For that you will need to have access to your blog code (which most blogging platforms offer). To do that you need to log in in your admin panel, choose the footer or side bar where you want to put the link and copy/paste the code that directory has provided. I don't think you can or if it makes any sence to do that here on Hubpages.
Now about Directories in general. For a very long time submitting to a directory has little to no effect for your search ranking. There are of course some directories like DMOZ and others that have huge impact on that. I personally avoid most directories that require a link back. Why? Because it is an unfair exhange - I will link from unique content, with very few links and usually multiple pages while I will get a link burried on page 135 of some subcategory with another 15 links that on top of that will probably NOT be even indexed. So no, thank you. I do submit to directories that bring me TRAFFIC and don't hope that much on the link itself.
I am NOT SAYING you shouldn't submit to directories. I am saying to check them out. Here are some things you should be checking:
1. Will the link be followed by search engines? - Open the directory, select one of the non-paid links and check the code (select the link, right click, explore element / check code depending on the brawser you are using). If it contains "nofollow" in the code - don't bother submitting to them.
2. Where will your link be placed? Usually it will be the last page in the category/subcategory you have selected. Copy the URL and paste it in Google. If it doesn't give you the EXACT page in the search results - don't submit.
3. How many links are there per page? The lower the number, the better
4. Focus on NICHE directories. Try to find directories that are on your subject, or close to it. Directories that include links to pretty much everything are usually a bad bet. You will get better results this way.
If you want to get good Search Ranking results you should try getting links from different sources and most of all different types of links. Only directories will get you no where. Hope this helps.