Here’s how to add a widget of Chatimity( a place where all the nice people hang around chatting) to your tumblr blog:
1. Login to your Tumblr Dashboard.
2. Click on customize button (on the right hand column, below posts).
3. You’ll now be on a page which shows your theme with a left hand column.
4. On the left hand top, you’ll see a edit html link below your theme, click on that.
5. You’ll now have an editor open on the left column, this is your base template, hit Ctrl+F and type This is a html tag, just below this paste the following text.
Click on Update Preview button and then the save button that appears, once it is done reload the page.
If you want to choose a specific topic and want to change the name of the widget, change the topics= and title= in the iframe html tag above.