Some tablets from this developer don't support Google Play store in order to install applications but they also have additional market application where you can download them. Your Onda tablet supports Google Play store and all you need is a Google/Gmail account attached to the tablet in order to download and install applications.
To add an account enter Menu > Settings> Accounts & Sync > Choose Google and enter your details. If you don't have an account, you can create one using Google's main page.
Another way to install Yahoo Messenger is to download the apk installation package from a Google search using this term : Yahoo Messenger android apk + download then plug in the cable from the tablet to the PC and transfer the application to the storage device of the tablet.