It's easy to add Url Aliases and redirects in campaignion, for yoursite.campaignion.org you would go to:
Manage > Advanced Settings > Site settings
Search for URL Redirects or URL Aliases and click "add"
Difference between alias and redirect
alias lets you choose a nickname for your page, meaning a new url that points to the content the same way the orignial url does (that also means only donate.mencap.org/* urls are possible), while a redirect is a signpost to the next url, the url points not to the content but to the orignial url, which then points to the content.
Inside the CMS make sure to use only aliases, as combined usage of aliases and redirects can make very funny things, that mostly end up in 404 pages or endless loops ;-)