Count your blessings that it is not using all of the available memory. SQL Server will start at the min server memory when it starts up. As it performs more work (services new connections, reads index and data pages, creates/drops locks, etc.), it will acquire more memory from the OS up until it reaches the max server memory amount. If it does reach the max server memory limit, it will determine what items it can unload from memory (similar to Least Recently Used) to accommodate the new memory requirements.

That being said, if you have not yet reached the total memory available to SQL Server then you have enough memory in the machine. In my experience, having sufficient memory for SQL Server is the single biggest performance gain you achieve.