The regular RS client could have downloaded the cache files to the wrong spot. You can uninstall it and delete any old cache folders (locations like "C:\.jagex_cache_32", "C:\Users\username\.jagex_cache_32", "C:\Users\username\jagexcache", and any jagex files in "C:\Users\username"). Then launch the the RuneMate client and it should download everything again. You will have to do the video settings setup again.
If you have the .jar version (the standalone) make a text file and paste: java -jar -Xmx1024m RuneMate.jar
into it. Save the file into the same folder the runemate.jar file is located. Rename the file to runemate.bat (you might need to show file extensions in Windows to change it from .txt to .bat). Then double click it to launch RM. This will show a command prompt output so you can see errors and warnings.