This is a question for the people who have done this type of campaign (World War II). My Dungeon Master is interested in making a side campaign, he has a story set for World War II, but he's not really sure how to go about doing this, so here I am as the helpful player ^^
If there are some guides, books, or anything that he might need, if you guys can it link here, that would be really great. For example, one thing he wants to know is how to properly create modern technology, for example, a theatre, or a train. I tried the Useful D20 Modern links page, but most of the links did not work, though I did find the Official D20 Modern Character Sheet.
Thanks 
There is a dirth of material out there modern.
However, unlike trying to figure out what a catapult is like or what an archaic draw bridge/port is like and will work in game... we can just find out what Trains or Theaters really are like and stat them as that.
Cue up some DVDs of the period in question and stat stuff to fit what you see.
Fortunately, there is no shortage of WW2 movies / shows.
Of course, we here are more than able to help stat up stuff for you as well.
Are you looking at WW2 combat centered (frontlines) or the War Noir waged by the OSS in cafes & pianobars?
If you are thinking the frontlines,
I really REALLY suggest you look into my explosives formula for the heavy firepower,
and we can link some WW2 combat encounter/plothook threads
This is a question for the people who have done this type of campaign (World War II). My Dungeon Master is interested in making a side campaign, he has a story set for World War II, but he's not really sure how to go about doing this, so here I am as the helpful player ^^If there are some guides, books, or anything that he might need, if you guys can it link here, that would be really great. For example, one thing he wants to know is how to properly create modern technology, for example, a theatre, or a train. I tried the Useful D20 Modern links page, but most of the links did not work, though I did find the Official D20 Modern Character Sheet.
sure...start here >>> Beyond the Barbed Wire [World War II Material]
then consider RPGObjects line Blood and Guts
and SteelMajic Studios entry World War II Heroes
enjoy!
okay a two-fer .... try this also 
Pinnacle used to make a d20 game called "Weird War II". It's pre-Modern, uses 3.0 rules, and is generally good. I did some tweaking of the rules and ran it for my group. Worked pretty well.
This is a question for the people who have done this type of campaign (World War II). My Dungeon Master is interested in making a side campaign, he has a story set for World War II, but he's not really sure how to go about doing this, so here I am as the helpful player ^^If there are some guides, books, or anything that he might need, if you guys can it link here, that would be really great. For example, one thing he wants to know is how to properly create modern technology, for example, a theatre, or a train. I tried the Useful D20 Modern links page, but most of the links did not work, though I did find the Official D20 Modern Character Sheet.
and for background and hooks ...straight from headline news
I have a few of Tema69's homebrew works that revolve around WW II so if you are interested in viewing them contact me and I could send them to you.