Block IDs are numbers that Manic Digger uses to distinguish different blocks. The IDs that you choose must not only be different from each other, but also all other blocks in Manic Digger. So long as you have not downloaded any non-default mods, ID 300 and up should work just fine.
You can specify the name of a an image in the file @ ManicDigger/data/public/blocks to show on the sides of your blocks. Make sure to omit the end of the file (i.e. ".png"). For example, if you wanted to use an image in the textures folder named "empty.png" you would write "empty" in the blank. You can also add your own images to the textures folder, but make sure they are PNG images which are 32x32 pixels large.
Press the download button. If you filled out every necessary field your mod will start downloading. If nothing happens, refer to the Unanswered Questions & Unsolved Problems section bellow. Now that your mod has downloaded, all you have to do is copy it to the folder @ ManicDigger/Mods/Fortress. Next place your textures in the folder @ ManicDigger/data/public/blocks.
If you still have a question or a problem after reading all the above head over to the Manic Digger forum and ether post a message on the Mod Creator thread or send me (Mr. Rar) a message.