The trainer that is presented by VA is the one that is built into Windows (normally found by going into the Control Panel/Speech Recognition applet). A shortcut is available through VA to the trainer, however-
Go to the Options screen, over to the 'Recognition' tab.
Click on 'Utilities' and then 'Speech Engine Training'.
Work through the training steps.
Restart VA (might not be necessary, doesn't hurt).
On a side note, if you had elected to hide the tips screen, the startup tip that shows the shortcut to the trainer would also be hidden. An uninstall/reinstall would not fix that - you'd have to check the box, 'Show tips at startup' on the Options screen > General tab to bring that back.
Hope that helps!