How do I pass a simple string value to a template. The examples in the book suggest I should be able to add it as an element to the array passed to the customise function but nothing is being passed to the template.

For example if I add "MyString"=>"stringvalue" to the array sent to customise and then do <% if MyString == 'stringvalue' %> in my template nothing is happening as MyValue is not passed.

Any ideas as to what I am doing wrong?

I am using 2.4

Test if the value comes up in the template by just calling the variable as so:

$MyString

If it comes up, it works.

For the if statement, I don't think you are allowed to have quotes in there:
http://doc.silverstripe.org/templates#if_blocks

So your if statement should be:

<% if MyString == stringvalue %>

<% end_if %>