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The U6 works great with load cells.  We recommend using the DAC outputs to provide excitation.  They will droop under the load, but they are stable and low-noise.  The droop is not a big deal as you will use an analog input to monitor the excitation voltage in real time anyway, as should be done with any excitation source.

Set the DAC to 4.5 volts.  Perhaps use "config defaults" in LJControlPanel if you want this to be the power-up default.  The DAC outputs have 50 ohms of source impedance, so with one 350 ohm bridge, you will draw about 11 mA which will drop about 0.5 volts internally leaving 4 volts for the bridge.  Each DAC (the U6 has 2) should be good to excite perhaps 3 bridge circuits, so if you have more than 6 bridge circuits you will need to use VS or add an excitation source.

Since you feedback the excitation source to an analog input, VS (5 volts) works pretty good, but your data will be noisier than when using a DAC output.

No, the analog inputs are all multiplexed to a single ADC, or 2 ADCs for the U6-Pro.  See Section 2.0 for a system diagram and Section 3.2 for stream timing.

The high-resolution converter is not used in stream mode.  See Section 3 of the U6 User's Guide for related discussions.  Use LJLogUD to experiment with all resolution indexes from 1-12.

You don't need to add a reference to the Labjackud.dll since that .dll isn't a .NET component.  The reference you need is LJUDDotNet which should already be there if you are using one of our examples.

Most likely the error here is 64-bit related.  Some of the express editions will by default build the app for the architecture of the machine.  Thus, on a 32-bit machine it will make a 32-bit .exe which uses the 32-bit .dlls.  On a 64-bit machine it will build a 64-bit .exe and then require 64-bit .dlls.  It is probably giving that error because it can't find the 64-bit copy of LabJackUD.dll in the system32 directory (which is where Windows stores 64-bit .dlls).  You can get that file from here: http://labjack.com/support/ud/ud-setup-64-bit/labjackud-dll-64-bit .

Another option is to change the build settings for the project so even on a 64-bit PC it will build an x86 (32-bit) executable.  Many people prefer this option since it will work the same on all machines regardless of if they are 32-bit or 64-bit. LJControlPanel is done in this way as an x86 .NET application written in C# which would explain why it works but the one you build does not.

LJ_chALL_CHANNELS is used with LJ_ioGET_STREAM_DATA.  It refers to all channels that you specify when using LJ_ioADD_STREAM_CHANNEL.  Thus, if you add 2 channels, say AIN0 and AIN3, then LJ_chALL_CHANNELS would return data for both of them.  You can request just data from individual channels, but by far the most common option is to get all data back using this method.

1001 is an unknown error, which typically means some exception was thrown inside the driver.  The most common cause of this is a memory error of some sort, for instance the array being passed is not passed properly, or not allocated to be large enough to hold all the data being placed into it.  -999 is a value that is returned when there is an error of some sort so you know there is an error, it's just a check to ensure that the user is aware that something went wrong. 

I'm not sure what would cause the error 25, but if it is some memory error causing the 1001, then that could just be a result of that error.  It's best to figure out that one first, as the other will likely go away once it's fixed.

For the rest of your question, it depends on your code/language/etc.  I believe we have an email from you.  I will respond to that with some more information to gather that can help us track it down for you.