Jamie,
The latest trial install on the D-tools web site is 5.5 SP1 and this will install a SQL instance which is compatible with both 32 bit and 64 bit machines. If you are starting afresh with a trial I suggest you get the latest trial version.
Your other option is to download the 64 bit SQL Express - check this link on where to get it. This is a free download.
64 bit SQL Express
Once you install the 64 bit SQL Express start the SI 5.5 Server install and in your server configration wizard indicate that you have an existing SQL Server instance and select .\SQLEXPRESS as the SQL instance (this means the default SQLExpress instance on your local machine). Note: Do NOT install a new SQL Server instance as you now already have a valid SQL instance running on your server.
If any of this does not make sense contact support.