Post the code you're having problems so we can look at it but first you need to convert it to "raw input form" so that it can be cut and paste in text mode. For example, I wanted to solve:

$\displaystyle r^2y''+ry'+r^2y=0$

Then I'd use the DSolve command and enter into Mathematica:

$\displaystyle \text{DSolve}\left[r^2 y''[r]+r y'[r]+r^2 y[r]==0,y,r\right]$

then execute the command. Now select the cell (click the bracket on the right margin), then choose Cell/Convert To/Raw Input form in the menu. Ok, now it's in a text form that can be cut and paste. Cut it and now paste it into the thread here like I did below. But I also enclosed the text in code commands to make it look nicer but you don't have to do that:

Code:

In[1]:=
DSolve[r^2*Derivative[2][y][r] +
r*Derivative[1][y][r] + r^2*y[r] == 0,
y, r]
Out[1]=
{{y -> Function[{r}, BesselJ[0, r]*C[1] +
BesselY[0, r]*C[2]]}}