How to assign the same value to multiple resistors in an LTSPICE circuit
You need to set the value of those resistors to R=R. So, where you used to set the value to a number without a preceding R=, now the value has to start with R= 

It may be more clear to use .param Rdefault=100 and R=Rdefault.

Make sure the .param Rdefault=100 is a spice directive, not a comment.

In answer to using curly braces { and }:
For both versions of LTspice (IV and XVIII) applies that when you want to assign a numerical value in the form of an equation, you have to use burly braces.
For example, when setting the value to {arctan(1)*4}.
In OP's question, setting the value to {Rdefault} will work too.

However, for non-numerical values, you need to (and for numerical values you can) precede the value with R= . When preceding it with R=, you do not need to use the burly braces:
E.g. when setting the value to R=arctan(1)*4*time

Therefore my suggestion to always use the preceding R=