You may get an issue with Truecrypt on your synology about the ioctl and the device mapper (dm). Do "dmesg" when you try and use truecrypt to mount an image and see if it complains about a missing module.
The issue is because the synology default kernel lacks the required device mapper crypto module and the crypto modules XTS and LRW.
You can compile these using an ubuntu/linux host and the Synology cross-compile tools and sources. See my post on compiling a kernel module.
Alternatively, if you are on a synology 1511+ or similar (running a x86 processor - do "uname -a" to check), leave me a comment and i will email you the pre-compiled modules)