A lot of that sounds really wrong.
First of all, using backslashes in strings - you need to escape each backslash character ('\\').  And the console will escape input for you, so you don't need it for user input, that's important. And I forgot. [/edit]
The extension is part of the file name, don't remove it.
You cannot take a variable like filename and glue it to a file path without code to concatenate the string. Concatenate is a real word, by the way.
If you have a string that is big enough you can do something like:
void RemoveNewlines(char *s)
char *nl = strrchr(s, '\n');
*nl = '\0';
char OpenMe[FILENAME_MAX] = "";
char enteredpath[FILENAME_MAX / 2];
char enteredname[FILENAME_MAX / 2];
FILE *fil = NULL;
printf("Please enter the file path:\n");
fgets(enteredpath, FILENAME_MAX / 2, stdin);
printf("Please enter the file name:\n");
fgets(enteredname, FILENAME_MAX / 2, stdin);
printf("Opening %s...\n", OpenMe);
fil = fopen(OpenMe, "r");
Please enter the file path:
Please enter the file name:
Opening C:\Users\Josh2\My Documents\hello.txt...
Of course, reading the path requires reading in spaces so scanf(%s", enteredpath); is not sufficient. Use fgets and remove any '\n' in the path.