I can find ZERO information on "LGDT418" on Google. Who is the manufacturer?
If it is LG - then can you link to their product on the web or supply some useful information on the product?
Here is the 100% scoop on what it sounds like:
You are in trouble.
Most HTIB (home theater in a box) systems offer almost zero upgrade path. You may not have any video inputs into the HTIB and not more than one digital audio input. You made a big mistake buying, without asking first.
I hate saying it, but do you have a receipt and can you return it and are you prepared for it simply not working?
The HTIB should have component video out. That output should be connected directly to your projector. That will get your DVD video going, no problem. The audio, should be setup as stated in the owners manual. No problem.
Other things? Well, you probably can't hook your HD cable up directly to the HTIB because it may not allow it, so you are SOL. Disconnect the component from the output of the HTIB, then plug it into the component output on the back of your cable box. Plug digital audio into the digital input on the HTIB. Switch your HTIB to the correct audio, now you should have cable.
Really, I am guessing, but I have seen this happen numerous times.
Please clarify the product so we can be sure, but it really sounds like you need to pull everything together with a receipt. Be ready to return it and do NOT pay a restocking fee if you explained in detail your needs to the sales associate.
There are HTIB setups that meet your needs - perhaps the Onkyo HTS780 system which isn't as sleek as others, but outperforms most for the money.
More info please.