Don't panic if the program doesn't accepts your input file.
Use the Logging facilities in Section 2.4 to find the exact location of the error.
Syntactic errors ("parse error") are easy to fix. Most times one forgot a comma, semicolon or brace, or one misspelled a keyword or floating point constant.
Semantic errors ("invalid expression") are more difficult to fix. Please refer to Chapter 6 to find out which nodes can be connected to the different node types (and in what order), and what values are allowed for them.