I take it you are trying to run them in UO 1.51. UO 1.51 only records and plays .dm_3 files, so you can't watch .dm_2 demos.
Unfortunately some of the UO demos on the site are .dm_2 files - this is because we did them before the 1.51 patch came out. I'm afraid unless you have an earlier patch version of UO you won't be able to view them.
If you are not on about UO and the patch, and you mean how do you watch .dm_2 files generally, load CoD/UO, open console, and type:
/demo <demoname>
If the demo file is called Carentan1.dm_2, then type this in console:
/demo Carentan1
(don't include the .dm_2 bit, also it is case sensitive).
Any problems still just ask
