First, a couple of simple checks - try removing the offending addon and see if OFP works normally. Check which verison of OFP you have against what the addon requires. If the addon needs Resistance and you don't have it, then you won't be able to use it without installing Resistance. Similarly, check which verison number of OFP you have installed - the number appears at the top of the main menu screen, and make sure that you have any patches needed to run the addon.
If you're still having problems, then read on...
DX stands for Direct X. This is part of Windows and is designed to allow programmers to create 3-D graphics and sophisticated sound effects. Since Audio and Graphics hardware develops very quickly, Direct X is updated often so that new hardware can be used to the full.
At the time OFP was released, the latest version of Direct X was 8. If you haven't got it installed, OFP won't work properly.
To check which version of Direct X you have installed, go to the Windows start menu and select the 'Run' option. In the box that appears, type 'dxdiag' and click OK. A window will appear and after a few seconds (or a minute if your PC is a bit old), some information about your computer will appear. The last item on the list tells you what version of Direct X you have.
If it says that you have version 7 or below, then the problem is easily solved.
If you have Windows 95 or 98: Insert your OFP installation CD (the original, not resistance). Close the installation menu and double click on your 'My Computer' icon. Right click on the appropriate CD drive and select 'Explore'. Make sure you close down any running programs and anti-virus software. Double click the 'Direct X' Folder. Then double click 'dxsetup.exe' and follow the installation instructions. This will install version 8 of Direct X and *should* solve the problem.
If you have Windows 98SE, ME, 2000 or XP: Go to
http://www.microsoft.com/downloads/details.aspx?FamilyID=0a9b6820-bfbb-4799-9908-d418cdeac197&displaylang=enand download and install the latest version, following all the instructions on the web page. Don't worry, Direct X updates are free.
If you try all that and are still stuck, please let me know.
Ade