There must be some way to navigate from a Post to a UserProfile. Use .Include() to pull in the necessary relationships and then access that. For example, if the Post has a navigable property called UserProfile, do the following.
var model = db.Posts
.Include( "UserProfile" )
.OrderByDescending(d => d.PostDate)
.Select(p => new PostViewModel
{
Id = p.Id,
Title = p.Title,
Link = p.Link,
Description = p.Description,
Username = p.UserProfile.UserName
}).ToList();