You need to have 10 messages in order to send or receive private messages. There is a "test" board under the technical support board where you can post repeatedly to get to the 10 messages. Here's a link to that:
https://www.disboards.com/forums/test-board.88/
To begin a new thread, there is a "post new thread" button towards the top of the page on the right, just below where your username is.
You might want to just post a new thread with your questions as there are a number of posters here that can help with information on power wheelchairs. There are a ton of helpful people on this board.