You will want to check to see if the subscription seat has yet to be assigned to you. You can do this by going to the Account Management Portal an check under the Members area. There you will see your available seat/s that have yet to be assigned and you will have the opportunity to assign it to yourself if needed.
https://ecom-prd.trimblepaas.com/ecom/portal#/subscription
Here is more information pertaining to the AMP
After the seat has been assigned to you, access to SketchUp should be as Dave described.
Hope this helps
CD